Hi Kratmel, Denis's idea is good, but I am also a little confused on what is being wanted. If the need is simply to measure the length of time the power has been off, and not actually do anything else, you could easily implement this using UTC.
Store the current RTC to UTC reading every second. Have a power up bit that activates a subroutine that compares the current UTC to your stored one, does maths that then displays results on a screen that tells you both the delay time and also flash an alert if it is over 15 minutes.
Or have a routine that always compares current UTC to the stored one, and if it goes over, or gets close etc does the same alert system. But without things being dependent on a power-up.
cheers, Aus