refactor(user): use consistent datetime standart within gRPC

cmd/ovpm/action_user.go

@@ -82,7 +82,7 @@ func userListAction(rpcServURLStr string) error {
 		}
 
 		createdAt := user.CreatedAt
-		if t, err := time.Parse(time.RubyDate, user.CreatedAt); err == nil {
+		if t, err := time.Parse(time.RFC3339, user.CreatedAt); err == nil {
 			createdAt = humanize.Time(t)
 		}
 

user.go

@@ -345,7 +345,7 @@ func (u *User) GetServerSerialNumber() string {
 
 // GetCreatedAt returns user's creation time.
 func (u *User) GetCreatedAt() string {
-	return u.CreatedAt.Format(time.UnixDate)
+	return u.CreatedAt.Format(time.RFC3339)
 }
 
 // getIP returns user's vpn ip addr.