I assume they just use the Gregorian calendar throughout - roughly what is called "proleptic Gregorian" [1], although since the Gregorian calendar started on 15

I assume they just use the Gregorian calendar throughout - roughly what is called "proleptic Gregorian" [1], although since the Gregorian calendar started on 15 October 1582, which is before 1601, it's not really proleptic. Just barely retroleptic. [1] https://en.wikipedia.org/wiki/Proleptic_Gregorian_calendar https://en.wikipedia.org/wiki/Proleptic_Gregorian_calendar