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xkcd #3228: Day Counter
(thelemmy.club)
Title text:
It has been −2,147,483,648 days since our last integer overflow.
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3228/
At a guess, the counter goes up by fractions of a day internally, but it's reset by subtracting the integer part, rounded.
The original version wouldn't have had the rounding but they "fixed" that after the first time they did a reset and it went to 1 not 0.
As to why they wouldn't just set it to zero, well here we have the joke under a microscope and it's struggling to stay alive. Put the scalpel down.