feat: Add TOML marshalling API

datetime.go

@@ -1,6 +1,7 @@
 package interpres
 
 import (
+	"fmt"
 	"regexp"
 	"strings"
 	"time"
@@ -22,6 +23,32 @@ 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 local date-time, e.g.
+// "1979-05-27T07:32:00" or "...:00.000000123" when the time has a fractional
+// second. The fractional component is zero-padded to nanosecond precision.
+func (ldt LocalDateTime) String() string {
+	base := ldt.Format("2006-01-02T15:04:05")
+	if ns := ldt.Nanosecond(); ns > 0 {
+		return base + "." + fmt.Sprintf("%09d", ns)
+	}
+	return base
+}
+
+// String returns the TOML-canonical rendering of the local date, e.g.
+// "1979-05-27".
+func (ld LocalDate) String() string { return ld.Format("2006-01-02") }
+
+// String returns the TOML-canonical rendering of the local time, e.g.
+// "07:32:00" or "...:00.000000123" when the time has a fractional second.
+// The fractional component is zero-padded to nanosecond precision.
+func (lt LocalTime) String() string {
+	base := lt.Format("15:04:05")
+	if ns := lt.Nanosecond(); ns > 0 {
+		return base + "." + fmt.Sprintf("%09d", ns)
+	}
+	return base
+}
+
 var (
 	offsetDateTimeLayouts = []string{
 		"2006-01-02T15:04:05.999999999Z07:00",