The simplest solution is to setup a timer that producess an interrupt at an interval suitable for your application. You create a main loop where you handle all your regulat stuff, let's say input and output. You also check if the timer interrupt has occurred, and , based on that, you handle all relevant time-related stuff in a separate part of your main-loop. You can create time granulation by adding (a) software prescaler(s). As an exemple take the following speudo-code that handles one action every 10 ms and one every 50 ms. You could expand it to suit any timing and any number of timed functions:
.
.
Setup10msTimer();
Prescale=0;
.
.
while(1) // main program loop
{
if (TimerInt) // Has an interrupt occurred?
{
Do10msThing();
if (Prescale>4) // >= 50 ms ?
{
Do50msThing();
Prescale=0;
}
TimerInt=0; // Wait for next 10 ms
}
}

_interrupt 10msTimer()
{
TimerInt=1;
Prescale++;
}