timestamp

hey,
i seen that topic about the logout after so many minutes thing and i
went on a php site and some guy said i shuld try using timestamps
but he wuldn't reply wen i asked him for an example could someone
give me an example or make a simple timestamp that will log the user out?
the way i want it is after so many minuts of doing nothing to log them out.

ive got the bit of the code the other guy posted in the
"logout user after so many minutes"

would someone make me a timestamp or give me an example!