Hm. Maybe you are using an older version of Windows?

On Windows XP Notepad has a menu entry Edit > Time/Date which inserts the current time and/or date depending on the locale- and date-settings. If that's not there, maybe the shortcut (F5) works in earlier notepads.

If that doesn't help either, you might want to try out Notepad2, an excellent replacement for Notepad.