diff --git a/.sampo/changesets/ardent-king-tursas.md b/.sampo/changesets/ardent-king-tursas.md
new file mode 100644
index 00000000..07afcc41
--- /dev/null
+++ b/.sampo/changesets/ardent-king-tursas.md
@@ -0,0 +1,5 @@
+---
+pypi/posthog: patch
+---
+
+Remove python-dateutil as a runtime dependency
diff --git a/mypy-baseline.txt b/mypy-baseline.txt
index 000341d1..0189ac4d 100644
--- a/mypy-baseline.txt
+++ b/mypy-baseline.txt
@@ -1,7 +1,7 @@
-posthog/utils.py:0: error: Library stubs not installed for "dateutil.tz"  [import-untyped]
 posthog/request.py:0: error: Library stubs not installed for "requests"  [import-untyped]
 posthog/request.py:0: note: Hint: "python3 -m pip install types-requests"
-posthog/request.py:0: error: Library stubs not installed for "dateutil.tz"  [import-untyped]
+posthog/request.py:0: note: (or run "mypy --install-types" to install all missing stub packages)
+posthog/request.py:0: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
 posthog/request.py:0: error: Incompatible types in assignment (expression has type "bytes", variable has type "str")  [assignment]
 posthog/consumer.py:0: error: Name "Empty" already defined (possibly by an import)  [no-redef]
 posthog/consumer.py:0: error: Need type annotation for "items" (hint: "items: list[<type>] = ...")  [var-annotated]
@@ -9,13 +9,7 @@ posthog/consumer.py:0: error: Unsupported operand types for <= ("int" and "str")
 posthog/consumer.py:0: note: Right operand is of type "int | str"
 posthog/consumer.py:0: error: Unsupported operand types for < ("str" and "int")  [operator]
 posthog/consumer.py:0: note: Left operand is of type "int | str"
-posthog/feature_flags.py:0: error: Library stubs not installed for "dateutil"  [import-untyped]
-posthog/feature_flags.py:0: error: Library stubs not installed for "dateutil.relativedelta"  [import-untyped]
 posthog/feature_flags.py:0: error: Unused "type: ignore" comment  [unused-ignore]
-posthog/client.py:0: error: Library stubs not installed for "dateutil.tz"  [import-untyped]
-posthog/client.py:0: note: Hint: "python3 -m pip install types-python-dateutil"
-posthog/client.py:0: note: (or run "mypy --install-types" to install all missing stub packages)
-posthog/client.py:0: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
 posthog/client.py:0: error: Name "queue" already defined (by an import)  [no-redef]
 posthog/client.py:0: error: Need type annotation for "queue"  [var-annotated]
 posthog/client.py:0: error: Incompatible types in assignment (expression has type "Any | list[Any]", variable has type "None")  [assignment]
diff --git a/posthog/client.py b/posthog/client.py
index 1b7f0a34..a4ffa1ed 100644
--- a/posthog/client.py
+++ b/posthog/client.py
@@ -3,11 +3,10 @@
 import os
 import sys
 import warnings
-from datetime import datetime, timedelta
+from datetime import datetime, timedelta, timezone
 from typing import Any, Dict, Optional, Union
 from uuid import uuid4
 
-from dateutil.tz import tzutc
 from typing_extensions import Unpack
 
 from posthog.args import ID_TYPES, ExceptionArg, OptionalCaptureArgs, OptionalSetArgs
@@ -1100,7 +1099,7 @@ def _enqueue(self, msg, disable_geoip):
 
         timestamp = msg["timestamp"]
         if timestamp is None:
-            timestamp = datetime.now(tz=tzutc())
+            timestamp = datetime.now(tz=timezone.utc)
 
         # add common
         timestamp = guess_timezone(timestamp)
@@ -1277,7 +1276,7 @@ def _load_feature_flags(self):
                     self._update_flag_state(
                         cached_data, old_flags_by_key=self.feature_flags_by_key or {}
                     )
-                    self._last_feature_flag_poll = datetime.now(tz=tzutc())
+                    self._last_feature_flag_poll = datetime.now(tz=timezone.utc)
                     return
                 else:
                     # Emergency fallback: if cache is empty and we have no flags, fetch anyway.
@@ -1324,7 +1323,7 @@ def _fetch_feature_flags_from_api(self):
                 self.log.debug(
                     "[FEATURE FLAGS] Flags not modified (304), using cached data"
                 )
-                self._last_feature_flag_poll = datetime.now(tz=tzutc())
+                self._last_feature_flag_poll = datetime.now(tz=timezone.utc)
                 return
 
             if response.data is None:
@@ -1395,7 +1394,7 @@ def _fetch_feature_flags_from_api(self):
             )
             self.log.warning(e)
 
-        self._last_feature_flag_poll = datetime.now(tz=tzutc())
+        self._last_feature_flag_poll = datetime.now(tz=timezone.utc)
 
     def load_feature_flags(self):
         """
diff --git a/posthog/feature_flags.py b/posthog/feature_flags.py
index edfdbef9..dda34053 100644
--- a/posthog/feature_flags.py
+++ b/posthog/feature_flags.py
@@ -1,3 +1,4 @@
+import calendar
 import datetime
 import hashlib
 import logging
@@ -5,9 +6,6 @@
 import warnings
 from typing import Optional
 
-from dateutil import parser
-from dateutil.relativedelta import relativedelta
-
 from posthog import utils
 from posthog.types import FlagValue
 from posthog.utils import convert_to_datetime_aware, is_valid_regex
@@ -530,7 +528,7 @@ def compare(lhs, rhs, operator):
             parsed_date = relative_date_parse_for_feature_flag_matching(str(value))
 
             if not parsed_date:
-                parsed_date = parser.parse(str(value))
+                parsed_date = parse_datetime(str(value))
                 parsed_date = convert_to_datetime_aware(parsed_date)
         except Exception as e:
             raise InconclusiveMatchError(
@@ -555,7 +553,7 @@ def compare(lhs, rhs, operator):
                 return override_value > parsed_date.date()
         elif isinstance(override_value, str):
             try:
-                override_date = parser.parse(override_value)
+                override_date = parse_datetime(override_value)
                 override_date = convert_to_datetime_aware(override_date)
                 if operator == "is_date_before":
                     return override_date < parsed_date
@@ -776,6 +774,40 @@ def match_property_group(
         return property_group_type == "AND"
 
 
+def parse_datetime(value: str) -> datetime.datetime:
+    text = value.strip()
+    if text.endswith("Z"):
+        text = text[:-1] + "+00:00"
+    elif text.upper().endswith(" UTC"):
+        text = text[:-4] + "+00:00"
+    elif re.fullmatch(r"\d{4}", text):
+        now = datetime.datetime.now()
+        return datetime.datetime(int(text), now.month, now.day)
+
+    text = re.sub(r" ([+-]\d{2}:\d{2})$", r"\1", text)
+    return datetime.datetime.fromisoformat(text)
+
+
+# Python's stdlib doesn't provide a calendar-aware month/year delta.
+# Clamp the day to the target month's end to match dateutil.relativedelta behavior.
+def _subtract_months(dt: datetime.datetime, months: int) -> Optional[datetime.datetime]:
+    month_index = dt.year * 12 + dt.month - 1 - months
+    year = month_index // 12
+    month = month_index % 12 + 1
+    if not 1 <= year <= 9999:
+        return None
+    day = min(dt.day, calendar.monthrange(year, month)[1])
+    return dt.replace(year=year, month=month, day=day)
+
+
+def _subtract_years(dt: datetime.datetime, years: int) -> Optional[datetime.datetime]:
+    year = dt.year - years
+    if not 1 <= year <= 9999:
+        return None
+    day = min(dt.day, calendar.monthrange(year, dt.month)[1])
+    return dt.replace(year=year, day=day)
+
+
 def relative_date_parse_for_feature_flag_matching(
     value: str,
 ) -> Optional[datetime.datetime]:
@@ -791,15 +823,15 @@ def relative_date_parse_for_feature_flag_matching(
 
         interval = match.group("interval")
         if interval == "h":
-            parsed_dt = parsed_dt - relativedelta(hours=number)
+            parsed_dt = parsed_dt - datetime.timedelta(hours=number)
         elif interval == "d":
-            parsed_dt = parsed_dt - relativedelta(days=number)
+            parsed_dt = parsed_dt - datetime.timedelta(days=number)
         elif interval == "w":
-            parsed_dt = parsed_dt - relativedelta(weeks=number)
+            parsed_dt = parsed_dt - datetime.timedelta(weeks=number)
         elif interval == "m":
-            parsed_dt = parsed_dt - relativedelta(months=number)
+            return _subtract_months(parsed_dt, number)
         elif interval == "y":
-            parsed_dt = parsed_dt - relativedelta(years=number)
+            return _subtract_years(parsed_dt, number)
         else:
             return None
 
diff --git a/posthog/request.py b/posthog/request.py
index fa54fac7..449f0bc4 100644
--- a/posthog/request.py
+++ b/posthog/request.py
@@ -9,7 +9,6 @@
 from typing import Any, List, Optional, Tuple, Union
 
 import requests
-from dateutil.tz import tzutc
 from requests.adapters import HTTPAdapter  # type: ignore[import-untyped]
 from urllib3.connection import HTTPConnection
 from urllib3.util.retry import Retry
@@ -197,7 +196,7 @@ def post(
     """Post the `kwargs` to the API"""
     log = logging.getLogger("posthog")
     body = kwargs
-    body["sentAt"] = datetime.now(tz=tzutc()).isoformat()
+    body["sentAt"] = datetime.now(tz=timezone.utc).isoformat()
     trimmed_host = remove_trailing_slash(normalize_host(host))
     url = trimmed_host + path
     body["api_key"] = api_key
diff --git a/posthog/test/test_feature_flags.py b/posthog/test/test_feature_flags.py
index 0a8269a5..0ce5e72c 100644
--- a/posthog/test/test_feature_flags.py
+++ b/posthog/test/test_feature_flags.py
@@ -2,7 +2,9 @@
 import unittest
 
 from unittest import mock
-from dateutil import parser, tz
+from zoneinfo import ZoneInfo
+from dateutil import parser
+from dateutil.relativedelta import relativedelta
 from freezegun import freeze_time
 from parameterized import parameterized
 
@@ -10,6 +12,7 @@
 from posthog.feature_flags import (
     InconclusiveMatchError,
     match_property,
+    parse_datetime,
     relative_date_parse_for_feature_flag_matching,
 )
 from posthog.request import APIError, GetResponse
@@ -4107,12 +4110,22 @@ def test_match_property_date_operators(self):
                 property_a,
                 {
                     "key": datetime.datetime(
-                        2022, 4, 30, 1, 2, 3, tzinfo=tz.gettz("Europe/Madrid")
+                        2022,
+                        4,
+                        30,
+                        1,
+                        2,
+                        3,
+                        tzinfo=ZoneInfo("Europe/Madrid"),
                     )
                 },
             )
         )
-        self.assertTrue(match_property(property_a, {"key": parser.parse("2022-04-30")}))
+        self.assertTrue(
+            match_property(
+                property_a, {"key": datetime.datetime.fromisoformat("2022-04-30")}
+            )
+        )
         self.assertFalse(match_property(property_a, {"key": "2022-05-30"}))
 
         # Can't be a number
@@ -4131,7 +4144,11 @@ def test_match_property_date_operators(self):
         self.assertTrue(
             match_property(property_b, {"key": datetime.datetime(2022, 5, 30)})
         )
-        self.assertTrue(match_property(property_b, {"key": parser.parse("2022-05-30")}))
+        self.assertTrue(
+            match_property(
+                property_b, {"key": datetime.datetime.fromisoformat("2022-05-30")}
+            )
+        )
         self.assertFalse(match_property(property_b, {"key": "2022-04-30"}))
 
         # can't be invalid string
@@ -4192,12 +4209,22 @@ def test_match_property_relative_date_operators(self):
                 property_a,
                 {
                     "key": datetime.datetime(
-                        2022, 4, 30, 1, 2, 3, tzinfo=tz.gettz("Europe/Madrid")
+                        2022,
+                        4,
+                        30,
+                        1,
+                        2,
+                        3,
+                        tzinfo=ZoneInfo("Europe/Madrid"),
                     )
                 },
             )
         )
-        self.assertTrue(match_property(property_a, {"key": parser.parse("2022-04-30")}))
+        self.assertTrue(
+            match_property(
+                property_a, {"key": datetime.datetime.fromisoformat("2022-04-30")}
+            )
+        )
         self.assertFalse(match_property(property_a, {"key": "2022-05-30"}))
 
         # Can't be a number
@@ -4214,7 +4241,11 @@ def test_match_property_relative_date_operators(self):
         self.assertTrue(
             match_property(property_b, {"key": datetime.datetime(2022, 5, 30)})
         )
-        self.assertTrue(match_property(property_b, {"key": parser.parse("2022-05-30")}))
+        self.assertTrue(
+            match_property(
+                property_b, {"key": datetime.datetime.fromisoformat("2022-05-30")}
+            )
+        )
         self.assertFalse(match_property(property_b, {"key": "2022-04-30"}))
 
         # can't be invalid string
@@ -4548,6 +4579,23 @@ def test_unknown_operator(self):
         )
 
 
+class TestDateParsing(unittest.TestCase):
+    @parameterized.expand(
+        [
+            ("iso_date", "2022-05-01"),
+            ("iso_datetime_Z", "2022-04-05T12:34:12Z"),
+            ("iso_datetime_utc", "2022-04-05 12:34:12 UTC"),
+            ("iso_datetime_offset", "2022-04-05 12:34:12 +01:00"),
+        ]
+    )
+    def test_parse_datetime_matches_dateutil_for_supported_formats(self, _name, value):
+        assert parse_datetime(value) == parser.parse(value)
+
+    def test_parse_datetime_year_only_matches_previous_dateutil_behavior(self):
+        with freeze_time("2020-04-03T00:00:00"):
+            assert parse_datetime("1234") == parser.parse("1234")
+
+
 class TestRelativeDateParsing(unittest.TestCase):
     def test_invalid_input(self):
         with freeze_time("2020-01-01T12:01:20.1340Z"):
@@ -4568,6 +4616,7 @@ def test_invalid_input(self):
 
     def test_overflow(self):
         assert relative_date_parse_for_feature_flag_matching("1000000h") is None
+        assert relative_date_parse_for_feature_flag_matching("9999y") is None
         assert (
             relative_date_parse_for_feature_flag_matching("100000000000000000y") is None
         )
@@ -4577,27 +4626,27 @@ def test_hour_parsing(self):
             assert relative_date_parse_for_feature_flag_matching(
                 "1h"
             ) == datetime.datetime(
-                2020, 1, 1, 11, 1, 20, 134000, tzinfo=tz.gettz("UTC")
+                2020, 1, 1, 11, 1, 20, 134000, tzinfo=datetime.timezone.utc
             )
             assert relative_date_parse_for_feature_flag_matching(
                 "2h"
             ) == datetime.datetime(
-                2020, 1, 1, 10, 1, 20, 134000, tzinfo=tz.gettz("UTC")
+                2020, 1, 1, 10, 1, 20, 134000, tzinfo=datetime.timezone.utc
             )
             assert relative_date_parse_for_feature_flag_matching(
                 "24h"
             ) == datetime.datetime(
-                2019, 12, 31, 12, 1, 20, 134000, tzinfo=tz.gettz("UTC")
+                2019, 12, 31, 12, 1, 20, 134000, tzinfo=datetime.timezone.utc
             )
             assert relative_date_parse_for_feature_flag_matching(
                 "30h"
             ) == datetime.datetime(
-                2019, 12, 31, 6, 1, 20, 134000, tzinfo=tz.gettz("UTC")
+                2019, 12, 31, 6, 1, 20, 134000, tzinfo=datetime.timezone.utc
             )
             assert relative_date_parse_for_feature_flag_matching(
                 "48h"
             ) == datetime.datetime(
-                2019, 12, 30, 12, 1, 20, 134000, tzinfo=tz.gettz("UTC")
+                2019, 12, 30, 12, 1, 20, 134000, tzinfo=datetime.timezone.utc
             )
 
             assert relative_date_parse_for_feature_flag_matching(
@@ -4612,27 +4661,27 @@ def test_day_parsing(self):
             assert relative_date_parse_for_feature_flag_matching(
                 "1d"
             ) == datetime.datetime(
-                2019, 12, 31, 12, 1, 20, 134000, tzinfo=tz.gettz("UTC")
+                2019, 12, 31, 12, 1, 20, 134000, tzinfo=datetime.timezone.utc
             )
             assert relative_date_parse_for_feature_flag_matching(
                 "2d"
             ) == datetime.datetime(
-                2019, 12, 30, 12, 1, 20, 134000, tzinfo=tz.gettz("UTC")
+                2019, 12, 30, 12, 1, 20, 134000, tzinfo=datetime.timezone.utc
             )
             assert relative_date_parse_for_feature_flag_matching(
                 "7d"
             ) == datetime.datetime(
-                2019, 12, 25, 12, 1, 20, 134000, tzinfo=tz.gettz("UTC")
+                2019, 12, 25, 12, 1, 20, 134000, tzinfo=datetime.timezone.utc
             )
             assert relative_date_parse_for_feature_flag_matching(
                 "14d"
             ) == datetime.datetime(
-                2019, 12, 18, 12, 1, 20, 134000, tzinfo=tz.gettz("UTC")
+                2019, 12, 18, 12, 1, 20, 134000, tzinfo=datetime.timezone.utc
             )
             assert relative_date_parse_for_feature_flag_matching(
                 "30d"
             ) == datetime.datetime(
-                2019, 12, 2, 12, 1, 20, 134000, tzinfo=tz.gettz("UTC")
+                2019, 12, 2, 12, 1, 20, 134000, tzinfo=datetime.timezone.utc
             )
 
             assert relative_date_parse_for_feature_flag_matching(
@@ -4644,28 +4693,28 @@ def test_week_parsing(self):
             assert relative_date_parse_for_feature_flag_matching(
                 "1w"
             ) == datetime.datetime(
-                2019, 12, 25, 12, 1, 20, 134000, tzinfo=tz.gettz("UTC")
+                2019, 12, 25, 12, 1, 20, 134000, tzinfo=datetime.timezone.utc
             )
             assert relative_date_parse_for_feature_flag_matching(
                 "2w"
             ) == datetime.datetime(
-                2019, 12, 18, 12, 1, 20, 134000, tzinfo=tz.gettz("UTC")
+                2019, 12, 18, 12, 1, 20, 134000, tzinfo=datetime.timezone.utc
             )
             assert relative_date_parse_for_feature_flag_matching(
                 "4w"
             ) == datetime.datetime(
-                2019, 12, 4, 12, 1, 20, 134000, tzinfo=tz.gettz("UTC")
+                2019, 12, 4, 12, 1, 20, 134000, tzinfo=datetime.timezone.utc
             )
             assert relative_date_parse_for_feature_flag_matching(
                 "8w"
             ) == datetime.datetime(
-                2019, 11, 6, 12, 1, 20, 134000, tzinfo=tz.gettz("UTC")
+                2019, 11, 6, 12, 1, 20, 134000, tzinfo=datetime.timezone.utc
             )
 
             assert relative_date_parse_for_feature_flag_matching(
                 "1m"
             ) == datetime.datetime(
-                2019, 12, 1, 12, 1, 20, 134000, tzinfo=tz.gettz("UTC")
+                2019, 12, 1, 12, 1, 20, 134000, tzinfo=datetime.timezone.utc
             )
             assert relative_date_parse_for_feature_flag_matching(
                 "4w"
@@ -4676,28 +4725,28 @@ def test_month_parsing(self):
             assert relative_date_parse_for_feature_flag_matching(
                 "1m"
             ) == datetime.datetime(
-                2019, 12, 1, 12, 1, 20, 134000, tzinfo=tz.gettz("UTC")
+                2019, 12, 1, 12, 1, 20, 134000, tzinfo=datetime.timezone.utc
             )
             assert relative_date_parse_for_feature_flag_matching(
                 "2m"
             ) == datetime.datetime(
-                2019, 11, 1, 12, 1, 20, 134000, tzinfo=tz.gettz("UTC")
+                2019, 11, 1, 12, 1, 20, 134000, tzinfo=datetime.timezone.utc
             )
             assert relative_date_parse_for_feature_flag_matching(
                 "4m"
             ) == datetime.datetime(
-                2019, 9, 1, 12, 1, 20, 134000, tzinfo=tz.gettz("UTC")
+                2019, 9, 1, 12, 1, 20, 134000, tzinfo=datetime.timezone.utc
             )
             assert relative_date_parse_for_feature_flag_matching(
                 "8m"
             ) == datetime.datetime(
-                2019, 5, 1, 12, 1, 20, 134000, tzinfo=tz.gettz("UTC")
+                2019, 5, 1, 12, 1, 20, 134000, tzinfo=datetime.timezone.utc
             )
 
             assert relative_date_parse_for_feature_flag_matching(
                 "1y"
             ) == datetime.datetime(
-                2019, 1, 1, 12, 1, 20, 134000, tzinfo=tz.gettz("UTC")
+                2019, 1, 1, 12, 1, 20, 134000, tzinfo=datetime.timezone.utc
             )
             assert relative_date_parse_for_feature_flag_matching(
                 "12m"
@@ -4706,45 +4755,60 @@ def test_month_parsing(self):
         with freeze_time("2020-04-03T00:00:00"):
             assert relative_date_parse_for_feature_flag_matching(
                 "1m"
-            ) == datetime.datetime(2020, 3, 3, 0, 0, 0, tzinfo=tz.gettz("UTC"))
+            ) == datetime.datetime(2020, 3, 3, 0, 0, 0, tzinfo=datetime.timezone.utc)
             assert relative_date_parse_for_feature_flag_matching(
                 "2m"
-            ) == datetime.datetime(2020, 2, 3, 0, 0, 0, tzinfo=tz.gettz("UTC"))
+            ) == datetime.datetime(2020, 2, 3, 0, 0, 0, tzinfo=datetime.timezone.utc)
             assert relative_date_parse_for_feature_flag_matching(
                 "4m"
-            ) == datetime.datetime(2019, 12, 3, 0, 0, 0, tzinfo=tz.gettz("UTC"))
+            ) == datetime.datetime(2019, 12, 3, 0, 0, 0, tzinfo=datetime.timezone.utc)
             assert relative_date_parse_for_feature_flag_matching(
                 "8m"
-            ) == datetime.datetime(2019, 8, 3, 0, 0, 0, tzinfo=tz.gettz("UTC"))
+            ) == datetime.datetime(2019, 8, 3, 0, 0, 0, tzinfo=datetime.timezone.utc)
 
             assert relative_date_parse_for_feature_flag_matching(
                 "1y"
-            ) == datetime.datetime(2019, 4, 3, 0, 0, 0, tzinfo=tz.gettz("UTC"))
+            ) == datetime.datetime(2019, 4, 3, 0, 0, 0, tzinfo=datetime.timezone.utc)
             assert relative_date_parse_for_feature_flag_matching(
                 "12m"
             ) == relative_date_parse_for_feature_flag_matching("1y")
 
+    def test_month_parsing_clamps_to_target_month_end(self):
+        for value in ("2020-03-31T00:00:00Z", "2021-03-31T00:00:00Z"):
+            with freeze_time(value):
+                now = datetime.datetime.now(datetime.timezone.utc)
+                assert relative_date_parse_for_feature_flag_matching(
+                    "1m"
+                ) == now - relativedelta(months=1)
+
+    def test_year_parsing_clamps_to_target_month_end(self):
+        with freeze_time("2020-02-29T00:00:00Z"):
+            now = datetime.datetime.now(datetime.timezone.utc)
+            assert relative_date_parse_for_feature_flag_matching(
+                "1y"
+            ) == now - relativedelta(years=1)
+
     def test_year_parsing(self):
         with freeze_time("2020-01-01T12:01:20.1340Z"):
             assert relative_date_parse_for_feature_flag_matching(
                 "1y"
             ) == datetime.datetime(
-                2019, 1, 1, 12, 1, 20, 134000, tzinfo=tz.gettz("UTC")
+                2019, 1, 1, 12, 1, 20, 134000, tzinfo=datetime.timezone.utc
             )
             assert relative_date_parse_for_feature_flag_matching(
                 "2y"
             ) == datetime.datetime(
-                2018, 1, 1, 12, 1, 20, 134000, tzinfo=tz.gettz("UTC")
+                2018, 1, 1, 12, 1, 20, 134000, tzinfo=datetime.timezone.utc
             )
             assert relative_date_parse_for_feature_flag_matching(
                 "4y"
             ) == datetime.datetime(
-                2016, 1, 1, 12, 1, 20, 134000, tzinfo=tz.gettz("UTC")
+                2016, 1, 1, 12, 1, 20, 134000, tzinfo=datetime.timezone.utc
             )
             assert relative_date_parse_for_feature_flag_matching(
                 "8y"
             ) == datetime.datetime(
-                2012, 1, 1, 12, 1, 20, 134000, tzinfo=tz.gettz("UTC")
+                2012, 1, 1, 12, 1, 20, 134000, tzinfo=datetime.timezone.utc
             )
 
 
diff --git a/posthog/test/test_utils.py b/posthog/test/test_utils.py
index b6907be1..ed19005a 100644
--- a/posthog/test/test_utils.py
+++ b/posthog/test/test_utils.py
@@ -2,12 +2,11 @@
 import time
 import unittest
 from dataclasses import dataclass
-from datetime import date, datetime, timedelta
+from datetime import date, datetime, timedelta, timezone
 from decimal import Decimal
 from typing import Optional
 from uuid import UUID
 
-from dateutil.tz import tzutc
 from parameterized import parameterized
 from pydantic import BaseModel
 from pydantic.v1 import BaseModel as BaseModelV1
@@ -30,13 +29,13 @@ def test_is_naive(self, _name: str, expected_naive: bool):
         if expected_naive:
             dt = datetime.now()  # naive datetime
         else:
-            dt = datetime.now(tz=tzutc())  # timezone-aware datetime
+            dt = datetime.now(tz=timezone.utc)  # timezone-aware datetime
 
         assert utils.is_naive(dt) is expected_naive
 
     def test_timezone_utils(self):
         now = datetime.now()
-        utcnow = datetime.now(tz=tzutc())
+        utcnow = datetime.now(tz=timezone.utc)
 
         fixed = utils.guess_timezone(now)
         assert utils.is_naive(fixed) is False
@@ -80,7 +79,7 @@ def test_clean(self):
     def test_clean_with_dates(self):
         dict_with_dates = {
             "birthdate": date(1980, 1, 1),
-            "registration": datetime.now(tz=tzutc()),
+            "registration": datetime.now(tz=timezone.utc),
         }
         assert dict_with_dates == utils.clean(dict_with_dates)
 
diff --git a/posthog/utils.py b/posthog/utils.py
index d1dbc7ea..8c28a091 100644
--- a/posthog/utils.py
+++ b/posthog/utils.py
@@ -13,8 +13,6 @@
 import platform
 import distro  # For Linux OS detection
 
-from dateutil.tz import tzlocal, tzutc
-
 log = logging.getLogger("posthog")
 
 
@@ -36,12 +34,12 @@ def guess_timezone(dt):
         # case, and then defaults to utc
         delta = datetime.now() - dt
         if total_seconds(delta) < 5:
-            # this was created using datetime.datetime.now()
-            # so we are in the local timezone
-            return dt.replace(tzinfo=tzlocal())
+            # this was created using datetime.datetime.now(),
+            # so use the current system local timezone
+            return dt.replace(tzinfo=datetime.now().astimezone().tzinfo)
         else:
             # at this point, the best we can do is guess UTC
-            return dt.replace(tzinfo=tzutc())
+            return dt.replace(tzinfo=timezone.utc)
 
     return dt
 
diff --git a/pyproject.toml b/pyproject.toml
index 4904b87f..869d9e19 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -25,7 +25,6 @@ classifiers = [
 ]
 dependencies = [
     "requests>=2.7,<3.0",
-    "python-dateutil>=2.2",
     "backoff>=1.10.0",
     "distro>=1.5.0",
     "typing-extensions>=4.2.0",
@@ -46,7 +45,6 @@ dev = [
     "lxml",
     "mypy",
     "mypy-baseline",
-    "types-python-dateutil",
     "types-requests",
     "types-setuptools",
     "pre-commit",
@@ -61,6 +59,8 @@ dev = [
 ]
 test = [
     "freezegun==1.5.1",
+    "python-dateutil>=2.9.0.post0",
+    "tzdata",
     "coverage",
     "pytest",
     "pytest-timeout",
diff --git a/uv.lock b/uv.lock
index c1d0bb9e..42be7134 100644
--- a/uv.lock
+++ b/uv.lock
@@ -8,7 +8,7 @@ resolution-markers = [
 ]
 
 [options]
-exclude-newer = "2026-04-17T19:08:13.029980724Z"
+exclude-newer = "2026-04-21T13:48:20.831231Z"
 exclude-newer-span = "P7D"
 
 [[package]]
@@ -2042,7 +2042,6 @@ source = { editable = "." }
 dependencies = [
     { name = "backoff" },
     { name = "distro" },
-    { name = "python-dateutil" },
     { name = "requests" },
     { name = "typing-extensions" },
 ]
@@ -2061,7 +2060,6 @@ dev = [
     { name = "tomli" },
     { name = "tomli-w" },
     { name = "twine" },
-    { name = "types-python-dateutil" },
     { name = "types-requests" },
     { name = "types-setuptools" },
     { name = "wheel" },
@@ -2094,7 +2092,9 @@ test = [
     { name = "pytest" },
     { name = "pytest-asyncio" },
     { name = "pytest-timeout" },
+    { name = "python-dateutil" },
     { name = "tiktoken" },
+    { name = "tzdata" },
 ]
 
 [package.dev-dependencies]
@@ -2136,7 +2136,7 @@ requires-dist = [
     { name = "pytest", marker = "extra == 'test'" },
     { name = "pytest-asyncio", marker = "extra == 'test'" },
     { name = "pytest-timeout", marker = "extra == 'test'" },
-    { name = "python-dateutil", specifier = ">=2.2" },
+    { name = "python-dateutil", marker = "extra == 'test'", specifier = ">=2.9.0.post0" },
     { name = "requests", specifier = ">=2.7,<3.0" },
     { name = "ruff", marker = "extra == 'dev'" },
     { name = "setuptools", marker = "extra == 'dev'" },
@@ -2144,10 +2144,10 @@ requires-dist = [
     { name = "tomli", marker = "extra == 'dev'" },
     { name = "tomli-w", marker = "extra == 'dev'" },
     { name = "twine", marker = "extra == 'dev'" },
-    { name = "types-python-dateutil", marker = "extra == 'dev'" },
     { name = "types-requests", marker = "extra == 'dev'" },
     { name = "types-setuptools", marker = "extra == 'dev'" },
     { name = "typing-extensions", specifier = ">=4.2.0" },
+    { name = "tzdata", marker = "extra == 'test'" },
     { name = "wheel", marker = "extra == 'dev'" },
 ]
 provides-extras = ["langchain", "otel", "dev", "test"]
@@ -3201,15 +3201,6 @@ wheels = [
     { url = "https://files.pythonhosted.org/packages/7c/b6/74e927715a285743351233f33ea3c684528a0d374d2e43ff9ce9585b73fe/twine-6.1.0-py3-none-any.whl", hash = "sha256:a47f973caf122930bf0fbbf17f80b83bc1602c9ce393c7845f289a3001dc5384", size = 40791, upload-time = "2025-01-21T18:45:24.584Z" },
 ]
 
-[[package]]
-name = "types-python-dateutil"
-version = "2.9.0.20250516"
-source = { registry = "https://pypi.org/simple" }
-sdist = { url = "https://files.pythonhosted.org/packages/ef/88/d65ed807393285204ab6e2801e5d11fbbea811adcaa979a2ed3b67a5ef41/types_python_dateutil-2.9.0.20250516.tar.gz", hash = "sha256:13e80d6c9c47df23ad773d54b2826bd52dbbb41be87c3f339381c1700ad21ee5", size = 13943, upload-time = "2025-05-16T03:06:58.385Z" }
-wheels = [
-    { url = "https://files.pythonhosted.org/packages/c5/3f/b0e8db149896005adc938a1e7f371d6d7e9eca4053a29b108978ed15e0c2/types_python_dateutil-2.9.0.20250516-py3-none-any.whl", hash = "sha256:2b2b3f57f9c6a61fba26a9c0ffb9ea5681c9b83e69cd897c6b5f668d9c0cab93", size = 14356, upload-time = "2025-05-16T03:06:57.249Z" },
-]
-
 [[package]]
 name = "types-pyyaml"
 version = "6.0.12.20250516"