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It's a signed integer. The overflow will make the number negative, not zero.

And because it's JS. Not every language defines signed integer overflow as wrapping.

Most languages assume twos complement signed numbers. So an uncaught overflow will be wrapped around

JS specifically won’t necessarily wrap at this point, because adding one to the Max 32 but signed int maybe be a float.

Assuming I understand your question correctly: If the counter is stored as a signed int32 then the rollover will be to the smallest representable signed 32 which corresponds to the 1901 date. It's stored signed because software (particularly old software) wanted to be able to manipulate pre 1970 dates.

I’d guess that it’s because it’s a signed int? So the value can be negative. Probably depends on the library implementation though.