You can change the time whenever you want and you can always change it back to whatever is current or current.
Like iceringer said, make sure that no icons are selected and then press the B button; this will bring the time screen up. From there, press A and C at the same time, and the word "SET" should come up just below there the time is displayed. Use B to scroll down to the "hour" section of the time; use A to scroll through the options, and B to "select" it. Do the same for the minutes, and you're done! To get back to your tamas regular screen, simply push B again.