Added and corrected comments

date.go

@@ -10,14 +10,19 @@ type Date struct {
 	DateTime string
 }
 
+// DateTime formats a time.Time to string formated as defined by RFC3339. It
+// returns a string.
 func DateTime(t time.Time) string {
 	return string(t.Format(time.RFC3339))
 }
 
+// NewDate creates a new Date. It returns a *Date.
 func NewDate(t time.Time) *Date {
 	return &Date{DateTime: DateTime(t)}
 }
 
+// Check checks the Date for incompatibilities with RFC4287. It returns an
+// error.
 func (d *Date) Check() error {
 	if d.DateTime == "" {
 		return errors.New("date time element of date is empty")