Add OffsetDateTime chapter

Author: bowbahdoe

src/time/offset_date_time.md

@@ -1 +1,63 @@
 # OffsetDateTime
+
+An `OffsetDateTime` is similar to a `ZonedDateTime` but with the key difference
+that an `OffsetDateTime` doesn't record a moment in a specific time zone, but instead
+as an offset from the [UTC](https://en.wikipedia.org/wiki/Coordinated_Universal_Time)[^utc] timezone.
+
+This is useful because timezones change their rules frequently. If you had to pick a representation
+of dates and times to store, this is a good default.
+
+```java
+import java.time.LocalDate;
+import java.time.LocalTime;
+import java.time.LocalDateTime;
+import java.time.ZonedDateTime;
+import java.time.ZoneId;
+
+class Main {
+    void main() {
+        var feb14 = LocalDate.of(2025, 2, 14);
+        var fiveTwentyThree = LocalTime.of(5, 23, 0);
+        var est = ZoneId.of("US/Eastern");
+
+        LocalDateTime localDateTime = LocalDateTime.of(feb14, fiveTwentyThree);
+        ZonedDateTime zonedDateTime = ZonedDateTime.of(localDateTime, est);
+        OffsetDateTime offsetDateTime = zonedDateTime.toOffsetDateTime();
+
+        System.out.println(offsetDateTime);
+    }
+}
+```
+
+You can get the current `OffsetDateTime` for the time zone your computer is running in
+with `OffsetDateTime.now()`.
+
+```java
+import java.time.OffsetDateTime;
+
+class Main {
+    void main() {
+        var now = OffsetDateTime.now();
+
+        System.out.println(now);
+    }
+}
+```
+
+And you can do the same for an arbitrary time zone by giving a `ZoneId` to
+`now`. Java knows the UTC offset for every time zone.
+
+```java
+import java.time.OffsetDateTime;
+import java.time.ZoneId;
+
+class Main {
+    void main() {
+        var now = OffsetDateTime.now(ZoneId.of("US/Eastern"));
+
+        System.out.println(now);
+    }
+}
+```
+
+[^utc]: UTC stands for "Coordinated Universal Time." No I don't know why the letters are in a different order [and at this point I'm too afraid to ask](https://knowyourmeme.com/memes/afraid-to-ask-andy).