add new user defined user exiration

models/user.go

@@ -15,3 +15,17 @@ type User struct {
 func (u *User) IsValid() bool {
 	return u.Name != ""
 }
+
+func (u *User) ExpirationIsValid() bool {
+	if u.Expiration == nil {
+		return true
+	}
+	return time.Now().Before(*u.Expiration)
+}
+
+func (u *User) GetExpiration() string {
+	if u.Expiration == nil {
+		return ""
+	}
+	return u.Expiration.Format(time.RFC3339)
+}