The RTC has two modes of operation, selectable through the clock_mode bit.
In Calendar mode, the RTC counts seconds, minutes, hours, days, month, years, weekdays, and weeks. In Binary mode, the RTC consecutively counts from 0 all the way to 243 - 1. In both the modes, the alarm event generation logic simply compares the content of the Alarm register with that of the RTC; when they are equal, the RTC_MATCH output is asserted.