Pleekh - great choice of watch there. IWC's Perpetual Calendars are mechanically pre-programmed and all calendar displays are synchronized. This means that ever night at midnight, when the calendar advance occurs - the watch will advance the day, date, Moonphase, month (if necessary) year (if necessary), decade (if necessary) ,and century (if necessary).
As you know, the moon phase is a synodic month - which does not evenly align with 28, 29, 30, or 31 days. A full lunar cycle is approximately 30.44 days - but we don't even need to care about this.
IWC will set your watch at the factory, to a given date of the most recent FULL moon, such as "WED, 23 MARCH 2016". You will never need to independently need to reset the moon phase on the watch for 577 years.
The gear ratio of the BP Perpetual is big enough to have a massive gear geometry (same as Portuguese) to accommodate an unrivaled lunar accuracy.
More info on this can be found at
www.iwc.com/en/collection/portugieser/IW5033/