Arduino Uno deep sleep wake-up with DS3231: from once an hour to several times

cattledog:
Why do you have two alarms? I think that the one set for every 30 seconds will be waking the Arduino and the alarm desired at the 5 minute intervals will be adjusted ahead and never reached.

Indeed ALM2 is not needed and the setup line for this alarm must be omitted.
But I assumed the following:

  1. in setup ALM1 is set for every 30th second in the minute
  2. in loop ALM1 is changed right after wake-up to wake again 5 minutes after the current minute.

Why would the alarm ALM1 set at the 5 minute intervals be adjusted ahead and never be reached?