Cookbook: Display Formatted Date and Time (#3158)

* Cookbook: Display Formatted Date and Time

* Update data/cookbook/display-formatted-date-time/00-stdlib.ml

---------

Co-authored-by: Cuihtlauac Alvarado <cuihtlauac@users.noreply.github.com>

Author: pm5

data/cookbook/display-formatted-date-time/00-stdlib.ml

@@ -0,0 +1,24 @@
+---
+packages: []
+discussion: |
+  - **Understanding `Unix.tm`:** The `Unix.tm` structure represents a local time. This structure includes fields like `tm_year`, `tm_mon`, and `tm_mday` for year, month, and day, respectively.
+  - **Understanding `Format`:** Because `Unix.tm` is an abstract type, we must define a pretty printer for it.  The are usually two kinds of pretty printer for a type: `pp` uses `Format.formatter` to print a value, and `show` simply converts the value to a string.
+---
+
+(* The `unix` library, which ships with OCaml's standard library, provides
+   functions to work with dates and times. *)
+let today: Unix.tm = Unix.localtime (Unix.time ());;
+
+(* The `Unix.tm` type represents date and time, but it lacks a pretty printer.
+   We can define one for it. *)
+let pp_tm ppf t =
+  Format.fprintf ppf "%4d-%02d-%02dT%02d:%02d:%02dZ" (t.Unix.tm_year + 1900)
+    (t.Unix.tm_mon + 1) t.Unix.tm_mday t.Unix.tm_hour t.Unix.tm_min
+    t.Unix.tm_sec;;
+
+(* Then define a function that converts `Unix.tm` to string. *)
+let show_tm = Format.asprintf "%a" pp_tm;;
+
+Format.printf "The current date and time is %a" pp_tm today;;
+
+print_endline ("The current date and time is " ^ show_tm today);;