How to update current frame title?

[Song Fang]

Hi,

I want to display the date and time on the frame title, so I add a line
into the .emacs file:
    (setq frame-title-frame '("GNU Emacs "
                              (:eval (format-time-string "%Y-%m-%d %R"))))

But it only updates when the frame is redsplayed.
Can I update the title using something like `force-mode-line-update'?

Thanks

Song