close trade after n bars

there has to be an easier way to close a trade after n bars than what i'm failing to do (string manipulation of datetime variable). how do i achieve this?

I subtract the rounded opening time from the current bar time and divide by the period multiplied by 60 seconds. So this gives the difference in seconds divided by the number of seconds in the bar. Simplified example: 1200/5*60 = 4. The key is in the rounding - whether to round up to the next period or down to the bar it was on.

thanks. no annoying string functions.