Add support for epoch time (%s) to Calendar strftime (#12380)

lib/elixir/lib/calendar.ex

@@ -464,6 +464,7 @@ defmodule Calendar do
   p      | "AM" or "PM" (noon is "PM", midnight as "AM")                           | AM, PM
   P      | "am" or "pm" (noon is "pm", midnight as "am")                           | am, pm
   q      | Quarter                                                                 | 1, 2, 3, 4
+  s      | Number of seconds since the Epoch, 1970-01-01 00:00:00+0000 (UTC)       | 1565888877
   S      | Second                                                                  | 00, 59, 60
   u      | Day of the week                                                         | 1 (Monday), 7 (Sunday)
   x      | Preferred date (without time) representation                            | 2018-10-17
@@ -804,6 +805,34 @@ defmodule Calendar do
     parse(rest, datetime, format_options, [result | acc])
   end
 
+  # Epoch time for DateTime with time zones
+  defp format_modifiers(
+         "s" <> rest,
+         _width,
+         _pad,
+         datetime = %{utc_offset: _utc_offset, std_offset: _std_offset},
+         format_options,
+         acc
+       ) do
+    result =
+      datetime
+      |> DateTime.shift_zone!("Etc/UTC")
+      |> NaiveDateTime.diff(~N[1970-01-01 00:00:00])
+      |> Integer.to_string()
+
+    parse(rest, datetime, format_options, [result | acc])
+  end
+
+  # Epoch time
+  defp format_modifiers("s" <> rest, _width, _pad, datetime, format_options, acc) do
+    result =
+      datetime
+      |> NaiveDateTime.diff(~N[1970-01-01 00:00:00])
+      |> Integer.to_string()
+
+    parse(rest, datetime, format_options, [result | acc])
+  end
+
   # +hhmm/-hhmm time zone offset from UTC (empty string if naive)
   defp format_modifiers(
          "z" <> rest,

lib/elixir/test/elixir/calendar_test.exs

@@ -336,6 +336,26 @@ defmodule CalendarTest do
       assert Calendar.strftime(~N[2019-08-15 17:07:57], "%010A") == "00Thursday"
     end
 
+    test "formats Epoch time with %s" do
+      assert Calendar.strftime(~N[2019-08-15 17:07:57], "%s") == "1565888877"
+
+      datetime = %DateTime{
+        year: 2019,
+        month: 8,
+        day: 15,
+        hour: 17,
+        minute: 7,
+        second: 57,
+        microsecond: {0, 0},
+        time_zone: "Europe/Berlin",
+        zone_abbr: "CET",
+        utc_offset: 3600,
+        std_offset: 0
+      }
+
+      assert Calendar.strftime(datetime, "%s") == "1565885277"
+    end
+
     test "formats datetime with all options and modifiers" do
       assert Calendar.strftime(
                ~U[2019-08-15 17:07:57.001Z],