From OpenSCADAWiki
The first method consists in decrement of the counter, set in the time interval value, in each cycle, and at the periodicity of the cycle to the value of <= 0, for example, in OpenSCADA this is implemented as follows:
if((tm_cnt-=1/f_frq) <= 0) { //Decrement
tm_cnt = 10; //Setting the counter to 10 seconds
//Perform other actions with the 10 seconds period
}