The difference between programmers and testers
The difference between programmers and testers
The difference between programmers and testers
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Also misses the edge case where sister was born on a leap day
Or maybe in a country that recently switched from the Julian calendar, adding the possibility of >12 months between birthdays as described by calendar.
https://github.com/eggert/tz/blob/main/calendars has entered the chat.
Also, as ever, relevant XKCD.