On the scale of the Moonphase disc, you will see "29,5" printed, as it takes that many days for a full lunar month (not 30). https://www.cuhren.de/produkt-Sammlungen/ballon-de-cartier/ If you would https://www.crwwatches.to/ keep the watch running every day, it would only need https://www.cuhren.de/ one correction every ten years.