feat: add OffsetDateTime, nesting limits and uniform Marshaler dispatch

Assisted-by: DeepSeek V4.1 Flash

datetime.go

@@ -11,9 +11,15 @@ import (
 	"time"
 )
 
-// TOML distinguishes four date-time kinds. interpres decodes an offset
-// date-time to a plain time.Time (it carries a zone), and uses the wrapper
-// types below for the local variants so callers can tell them apart.
+// TOML distinguishes four date-time kinds, and each has its own Go type:
+// OffsetDateTime for the offset kind, and the local wrappers below for the
+// three that carry no offset. A plain time.Time is accepted wherever an
+// offset date-time is, on both the encoding and the decoding side, so a
+// timestamp field does not have to name the wrapper.
+
+// OffsetDateTime is a TOML offset date-time, e.g. 1979-05-27T07:32:00-07:00.
+// The embedded time.Time is the instant, with the offset the document wrote.
+type OffsetDateTime struct{ time.Time }
 
 // LocalDateTime is a TOML local date-time with no offset, e.g.
 // 1979-05-27T07:32:00. The embedded time.Time is in UTC.
@@ -27,6 +33,12 @@ type LocalDate struct{ time.Time }
 // The embedded time.Time uses the zero date.
 type LocalTime struct{ time.Time }
 
+// String returns the TOML-canonical rendering of the offset date-time, e.g.
+// "1979-05-27T07:32Z" or "1979-05-27T07:32:00-07:00". The seconds appear only
+// when the value carries them, a fractional second drops its trailing zeros,
+// and an offset of zero is written "Z".
+func (odt OffsetDateTime) String() string { return offsetString(odt.Time) }
+
 // String returns the TOML-canonical rendering of the local date-time, e.g.
 // "1979-05-27T07:32" or "1979-05-27T07:32:00.5" when the time carries a
 // fractional second. TOML 1.1 makes the seconds optional, so they appear only
@@ -129,7 +141,7 @@ func parseDateTime(tok string) (any, bool) {
 	norm := strings.ToUpper(tok)
 	for _, layout := range offsetDateTimeLayouts {
 		if t, err := time.Parse(layout, norm); err == nil {
-			return t, true
+			return OffsetDateTime{t}, true
 		}
 	}
 	for _, layout := range localDateTimeLayouts {