Github user ckadner commented on the issue:

    https://github.com/apache/spark/pull/13652
  
    @davies ...just to confirm the desired behavior of these two test cases 
```from UTC timestamp``` and ```to UTC timestamp```. We are contending with 3 
time zones:
    
    * we are parsing ```Timestamp```s from ```String```s in any/all time zones 
in the Java world (```DateTimeTestUtils.ALL_TIMEZONES)```) so that they 
represent the human readable time in any/each of those time zones which we are 
iterating over (*)
    * but in the inner ```test``` method we are treating these timestamps 
```utc: String```as if they were created in ```UTC```
    * then we are converting these timestamps (of "mistaken identity") to yet 
another time zone ```tz: String``` (i.e. ```"UTC", "JST", "PST", 
"Asia/Shanghai"```) and generating the human readable time in the time zone of 
the for-loop (```DateTimeTestUtils.withDefaultTimeZone(tz)```)
    
    ```scala
      test("from UTC timestamp") {
        def test(utc: String, tz: String, expected: String): Unit = {
           
assert(toJavaTimestamp(fromUTCTime(fromJavaTimestamp(Timestamp.valueOf(utc)), 
tz)).toString
             === expected)
        }
    
        for (tz <- DateTimeTestUtils.ALL_TIMEZONES) {
          DateTimeTestUtils.withDefaultTimeZone(tz) {
            test("2011-12-25 09:00:00.123456", "UTC", "2011-12-25 
09:00:00.123456")
            test("2011-12-25 09:00:00.123456", "JST", "2011-12-25 
18:00:00.123456")
            test("2011-12-25 09:00:00.123456", "PST", "2011-12-25 
01:00:00.123456")
            test("2011-12-25 09:00:00.123456", "Asia/Shanghai", "2011-12-25 
17:00:00.123456")
          }
        }
        ...
    ```
    (*) Before your fix (without the for-loop) the third time zone involved was 
determined by the user's JVM default Timezone.
    
    If this 3-timezone behavior is what we want to test, then should you 
replace the two occurrences of ...
    ```scala
        DateTimeTestUtils.withDefaultTimeZone(TimeZone.getTimeZone("PST")) {
    ```
    with ...
    ```scala
        for (tz <- DateTimeTestUtils.ALL_TIMEZONES) {
          DateTimeTestUtils.withDefaultTimeZone(tz) {
    ```
    ... or would ```2016-03-13 02:00:00 PST``` and ```2016-11-06 01:00:00 
PST``` not work?
    
    ```scala
        DateTimeTestUtils.withDefaultTimeZone(TimeZone.getTimeZone("PST")) {
          // Daylight Saving Time
          test("2016-03-13 01:59:59", "PST", "2016-03-13 09:59:59.0")
          // 2016-03-13 02:00:00 PST does not exists
          test("2016-03-13 02:00:00", "PST", "2016-03-13 10:00:00.0")
          test("2016-03-13 03:00:00", "PST", "2016-03-13 10:00:00.0")
          test("2016-11-06 00:59:59", "PST", "2016-11-06 07:59:59.0")
          // 2016-11-06 01:00:00 PST could be 2016-11-06 08:00:00 UTC or 
2016-11-06 09:00:00 UTC
          test("2016-11-06 01:00:00", "PST", "2016-11-06 09:00:00.0")
          test("2016-11-06 01:59:59", "PST", "2016-11-06 09:59:59.0")
          test("2016-11-06 02:00:00", "PST", "2016-11-06 10:00:00.0")
        }
    ```


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