how to adjust a timer????
 

I am programming in c and using gcc in RedHat.My question is:How may I adjust a timer that counts down for a given time period?

Well, without more specifics I can only list some man pages:

man alarm -- send a signal to a process in x seconds
man pause -- wait for signal
man sleep -- wait x seconds
man select -- wait for input. With no input, useful as a fine-grained sleep
man fork -- create an additional process (useful to do something after a sleep/select)

What exactly did you have in mind?