All valid, but none of that changes the answers

Of course it does. Some places observe daylight savings, others do not.

Anything to do with time and dates is more difficult than you imagined. And that includes recursion.

Let's start with *simple* questions:

- how many milliseconds are there in a second?
- how many seconds are there in a minute?
- how many minutes are there in an hour?
- how many hours are there in a day?
- how many days are there in a month?
- how many months are there in a year?
- how many years are there in a century?

Assuming we are talking UTC (so no daylight savings or politicians changing timezones):

- 1000 milliseconds in a second
- 59, 60 or 61 seconds in a minute (leap seconds)
- 60 minutes in an hour
- 24 hours in a day (may be 23 or 25 if daylight saving is observed)
- between 27 and 31 days in a month.
- 12 month in a year*
- 100 years in a century

What pitfalls did I miss (step into)?

In the UK, 1752 had 10 fewer days. I've heard tales that the tax year boundary used to be the equinox, and rather than change the amount of tax gathered, they kept the tax year the same length. Currently the tax year boundary is April 5th/6th.

Practically you

*have* to include DST; hence 23/24/25 is correct.

Different definitions of days and lunar months, of various types - but they aren't part of the calendar.

*) assuming Gregorian year. A year as part of an ISO 8601 week date may last until January of the next Gregorian year and may have 13 or maybe even 14 (partial) months.

How narrow minded and parochial; a common fallacious assumption

One calendar has up to 19 months of 19 days - plus a 4 or 5 days that aren't in months.