Create slash command for &aoc countdown

bot/exts/advent_of_code/_cog.py

@@ -182,6 +182,25 @@ async def aoc_countdown(self, ctx: commands.Context) -> None:
             f"The next event will start <t:{next_aoc_timestamp}:R>."
         )
 
+    @aoc_slash_group.command(name="countdown", description="Return time left until next day")
+    @whitelist_override(channels=AOC_WHITELIST)
+    async def aoc_countdown_slash(self, interaction: discord.Interaction) -> None:
+        """Return time left until next day."""
+        if _helpers.is_in_advent():
+            tomorrow, _ = _helpers.time_left_to_est_midnight()
+            next_day_timestamp = int(tomorrow.timestamp())
+
+            await interaction.response.send_message(f"Day {tomorrow.day} starts <t:{next_day_timestamp}:R>.")
+            return
+
+        next_aoc, _ = _helpers.time_left_to_next_aoc()
+        next_aoc_timestamp = int(next_aoc.timestamp())
+
+        await interaction.response.send_message(
+            "The Advent of Code event is not currently running. "
+            f"The next event will start <t:{next_aoc_timestamp}:R>."
+        )
+
     @adventofcode_group.command(name="about", aliases=("ab", "info"), brief="Learn about Advent of Code")
     @whitelist_override(channels=AOC_WHITELIST)
     async def about_aoc(self, ctx: commands.Context) -> None: