Fix #80185: jdtounix() fails after 2037

There is no such thing as the "end of the unix epoch", and if it was,
it would certainly not be 2037-10-11T02:00:00. There is, however,
poten

Fix #80185: jdtounix() fails after 2037

There is no such thing as the "end of the unix epoch", and if it was,
it would certainly not be 2037-10-11T02:00:00. There is, however,
potential integer overflow which we need to avoid.

Closes GH-6288.

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