Feature Request: Use existing HA entities or calendar events as date source

[moerk-o]

Currently all dates in WhenHub must be entered manually. This means keeping WhenHub in sync with other systems (calendars, automations, sensors from other integrations) requires manual updates.

Requested feature

Allow using an existing Home Assistant entity or calendar entry as the date source for a WhenHub event — instead of (or alongside) manual date input. WhenHub then applies all its calculations on top: countdown, "is active", percentage complete, etc.

Source option 1 — HA entity

Any entity with a date or timestamp value can be linked as the date source (e.g. input_datetime, sensor with device_class: date/timestamp). For Trip events, start and end date can each be linked independently.

Source option 2 — Calendar entry

The user picks a specific event directly from one of their HA calendars (Google Calendar, CalDav, etc.). For Trip events this is especially useful since a calendar entry already carries both start and end date.

Use cases

• Astronomy integration provides "spring equinox" → WhenHub Milestone counts down to the first day of spring
• Entry from a birthday calendar → WhenHub Anniversary with all its calculations on top
• Flight sensor with departure and arrival times → WhenHub Trip with live countdown
• Any device_class: timestamp sensor → WhenHub calculations without manual date entry

Expected behavior

• Date source selectable per event: manual / entity / calendar entry
• All source types available in config flow and options flow
• If source becomes unavailable, WhenHub sensors become unavailable too
• For Trip: if end date ends up before start date, a HA Repairs issue is created with a clear explanation