Github user davies commented on the pull request:
https://github.com/apache/spark/pull/8400#issuecomment-134482339
@yhuai The Julian day of epoch of unix timestamp (1970-01-01 0:0:0) is
2440587.5, if we use integer for julian days, then it will be cut off at noon
(12pm) of a day. But the nanoseconds is counted from 12am, so they are
overlapped from 12am to 12pm. In Hive, it create a calendar from Julian day
(which will have hours as 12pm), it then re-set the hours from nanoseconds.
In this patch, we use a trick to shift Julian days by 0.5 day then rounding
0.5 down to zero (In Hive, it rounds 0.5 up to 1), then we can add these two
parts together.
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