I noticed this sentence in "Concurrency Programming Guide" (Dispatch Sources).

"When a computer goes to sleep, all timer dispatch sources are suspended. When the computer wakes up, those timer dispatch sources are automatically woken up as well. Depending on the configuration of the timer, pauses of this nature may affect when your timer fires next. If you set up your timer dispatch source using the dispatch_time function or the DISPATCH_TIME_NOW constant, the timer dispatch source uses the default system clock to determine when to fire. However, the default clock does not advance while the computer is asleep. By contrast, when you set up your timer dispatch source using the dispatch_walltime function, the timer dispatch source tracks its firing time to the wall clock time. This latter option is typically appropriate for timers whose firing interval is relatively large because it prevents there from being too much drift between event times."

So, probably dispatch_walltime can solve it...