Update DateTime String method to use Value()

Refactor the DateTime String method to use the Value() function instead of accessing the internal field directly, for better encapsulation and readability.

datetime.go

@@ -36,5 +36,5 @@ func (dt DateTime) Value() time.Time {
 // String satisfies the fmt.Stringer interface for the DateTime type
 // The date will returned as time.RFC3339 format
 func (dt DateTime) String() string {
-	return dt.dv.Format(time.RFC3339)
+	return dt.Value().Format(time.RFC3339)
 }