[OJFord]

Moving bank/festive holidays, first Monday of the year(, first work day of the year not Monday if that's NYD and bank holiday), lunar occasions.

'subtract a year' is imprecise and has many meanings, if what you want is 'same day, same month, previous year' then say that and do that, that's conceptually `date.year -= 1` not `date -= 1 year`, and will have this bug.

[chatmasta]

You could subtract 365.25 days, but then you're left with a new problem: just because you can amortize a leap day over four years doesn't mean that you get an extra 6 hours each year.

[OJFord]

And as I just learnt elsewhere in this thread, it would actually be three four-hundredths less than that anyway, i.e. 365.2425, since only one in four centenaries is a leap year.