Thanks, MDG. In Patek, instantaneous date change is a special function in a few perpetual calendars, e.g, 5207P - combination of instantaneous PC + MR + tourbillon.
By: joenghenry : December 31st, 2020-09:55
On the other hand, a "simpler" retrograde PC like 5159 though not instantaneous, does perform date + day change in a blink, shortly before 23:59 in my sample. Cheers, Henry