The documentation says that the built-in date-time functions give undefined

The documentation says that the built-in date-time functions give undefined
results for dates before 0000-01-01 and after 9999-12-31.  Change the
actually implementation so that the answer given is really NULL.  This also
avoids unnecessary hand-wringing over an signed integer overflow that might
otherwise occur when processing out-of-bound dates.

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