Well, if you change the date and time, your Tama doesn't really get older.... it won't grow into an adult faster for example.
To change the Tama's time press button B (centre button) and you get the time screen.
Then press & hold button A & C and you get the date flashing.
Use button A to scroll to the date/time you want, and button B to confirm/set and it moves onto the next section of the clock.
Hope that helps
Best, etc.
TamaMum