add DateTime module

.changeset/clean-trainers-tap.md

@@ -0,0 +1,34 @@
+---
+"effect": minor
+---
+
+add DateTime module
+
+The `DateTime` module provides functionality for working with time, including
+support for time zones and daylight saving time.
+
+It has two main data types: `DateTime.Utc` and `DateTime.Zoned`.
+
+A `DateTime.Utc` represents a time in Coordinated Universal Time (UTC), and
+a `DateTime.Zoned` contains both a UTC timestamp and a time zone.
+
+There is also a `CurrentTimeZone` service, for setting a time zone contextually.
+
+```ts
+import { DateTime, Effect } from "effect";
+
+Effect.gen(function* () {
+  // Get the current time in the current time zone
+  const now = yield* DateTime.nowInCurrentZone;
+
+  // Math functions are included
+  const tomorrow = DateTime.add(now, 1, "day");
+
+  // Convert to a different time zone
+  // The UTC portion of the `DateTime` is preserved and only the time zone is
+  // changed
+  const sydneyTime = tomorrow.pipe(
+    DateTime.unsafeSetZoneNamed("Australia/Sydney"),
+  );
+}).pipe(DateTime.withCurrentZoneNamed("America/New_York"));
+```

.changeset/red-bottles-cough.md

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+---
+"@effect/schema": patch
+---
+
+add schemas for working with the DateTime module