iwc PORTOFINO Mode D'emploi page 19

Moon phase setting 16 | 17
Waxing Full Waning New
moon moon moon moon
Although the watch has only a 12-hour display, the movement differentiates
between 12 noon and 12 midnight. Like the date, the moon phase moves
forward by one day at midnight.
If the watch has been stopped for a long period, the moon phase can be
adjusted to the current state of the moon. Pull out the crown to position 1
and move the hands forward past midnight. The moon phase always moves
forward by one day when the hands are moved forward past midnight.
To simplify the moon phase setting, you can move the moon phase forward
as many times as you like by adjusting the time forward and back but only
between 9 pm and 1 am. The moon phase cannot be adjusted backwards.
Procedure
Find the current age of the moon from the daily newspaper or another
source (e.g. the Internet, lunar calendar). Adjust the moon phase to the last
full moon or new moon prior to the current date. Then count the days from
the full moon or new moon to the current date and move the moon phase
forward by this number of days.