Github user ckadner commented on the issue:
https://github.com/apache/spark/pull/14398
@holdenk @davies
I agree that date, time, and timezone handling is best left to the experts
and leveraging [JSR-310](http://www.threeten.org/threetenbp/)
([SPARK-16788](https://issues.apache.org/jira/browse/SPARK-16788)) in the long
run would probably solve most of the problems that have been painstakingly
fixed or attempted to be fixed in `DateTimeUtils.scala`.
The change proposed in this PR makes for cleaner code but it also regresses
the fix for [SPARK-15613](https://issues.apache.org/jira/browse/SPARK-15613).
With this fix there are 4 incorrect `day -> millis -> day` conversions since
1970/01/01 just for `Europe/Moscow`:
```scala
test("15613 Incorrect days to millis conversion Europe/Moscow") {
DateTimeTestUtils.withDefaultTimeZone(TimeZone.getTimeZone("Europe/Moscow")) {
val badDays = (0 to 20000).filterNot{ d =>
d === millisToDays(daysToMillis(d))
}
val df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss z")
badDays.foreach { d =>
println(s"day: ${d}, date: ${df.format(new Date(daysToMillis(d)))}")
}
assert(badDays.isEmpty)
}
}
```
```console
TestFailedException: Vector(4108, 4473, 4838, 5204) was not empty
day: 4108, date: 1981-03-31 23:00:00 MSK
day: 4473, date: 1982-03-31 23:00:00 MSK
day: 4838, date: 1983-03-31 23:00:00 MSK
day: 5204, date: 1984-03-31 23:00:00 MSK
```
...and it increases the number of incorrectly converted days from 52 to
4036 when testing all time zones and dates between 1900 and 2020 ([see test
code snippet
here](https://github.com/apache/spark/pull/13652#issuecomment-226877084)).
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