[BUGFIX] Fix date conversion of neg timestamps

Date conversion of TCA fields with eval "date" or "datetime" is now also
handled correctly for dates before 1970, thus having a negative
timestamp. Timezone offset is now also applied for those negative
timestamps, in order to prevent erroneous data for dates before 1970.

Resolves: #73871
Releases: master, 7.6
Change-Id: I4de9911dec3de720992da137fe8afcb3ecbfdad7
Reviewed-on: https://review.typo3.org/47115


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Andreas Wolf <andreas.wolf@typo3.org>
Tested-by: Andreas Wolf <andreas.wolf@typo3.org>