Good point, Peter. Clearly recording in EEPROM the date/time you started this batch would be essential. But without a RTC, if you lose power, you don't know how much time elapsed before it came back.
That's true, and I was assuming that the duration of any outages was negligible compared to the accuracy required for the overall sequence. Granted that RTCs are well known things, but the OP is evidently an Arduino novice working against the clock, and I would have thought that any complexity that could be avoided should be.