You can never rely on the timers firing exactly as you request, so you always have to take the delta since the last timer and use that in your calculations.

This is a very basic issues with doing stuff on a timer, and not that hard to do properly. It shouldn't make a difference if your timer is called once per second, once per minute or once in 10 hours in the background, you always calculate based on the delta to the last time your timer fired.