Sure, but if in this case the number would have only indicated the month you have an issue way earlier than 100 actually, you already have an issue on month 13 when you would go back got 01 and now you are overriding the old one.

And it's been about 25 years since we had to increase the number of digits for a year.

budget_97.csv, budget_98.csv, budget_99.csv, budget_2000.csv

> It’s been about two thousand years since the number of months in a year has been increased.

What? What are you thinking of? The number of months in a year is always 12 or 13 in any calendar system because they start by reflecting the moon. If you mean the Christian calendar, it was fixed at 12 months to the year well over 2000 years ago. If you mean any calendar, it's probably been more like one year since the number of months in a year has been increased. 12 lunar months falls short of a solar year by about 11 days, so any given lunar calendar will generate an extra month about every three years, and there are lots of different lunar calendars.

(For example, the Chinese calendar occasionally repeats full months in order to keep the month of the year lined up with the season. Whenever this happens, there will be 13 months in the year, of which two share the same name.)