Add back tests and add some more notes

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/DateTimeUtils.scala

@@ -877,11 +877,17 @@ object DateTimeUtils {
         val hh = seconds / 3600
         val mm = seconds / 60 % 60
         val ss = seconds % 60
+        val ms = millisOfDay % 1000
         val calendar = Calendar.getInstance(tz)
-
-        calendar.set(year, month, day, hh, mm, ss)
-        calendar.getTime() // Set the timezone info
+        calendar.set(year, month - 1, day, hh, mm, ss)
+        calendar.set(Calendar.MILLISECOND, ms)
+        val date = calendar.getTime()
+        // TODO decide between return the offset from the calendar:
         guess = calendar.get(Calendar.ZONE_OFFSET) + calendar.get(Calendar.DST_OFFSET)
+        // And returning the difference between timestamps (end up broken in different ways)
+        // This way appears to return a non-DST value even when in DST - but our round trips tests
+        // will work.
+        // guess = (millisLocal - calendar.getTimeInMillis()).toInt
       }
     }
     guess

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/util/DateTimeUtilsSuite.scala

@@ -536,6 +536,29 @@ class DateTimeUtilsSuite extends SparkFunSuite {
     }
   }
 
+  test("daysToMillis and millisToDays") {
+    // There are some days are skipped entirely in some timezone, skip them here.
+    val skipped_days = Map[String, Int](
+      "Kwajalein" -> 8632,
+      "Pacific/Apia" -> 15338,
+      "Pacific/Enderbury" -> 9131,
+      "Pacific/Fakaofo" -> 15338,
+      "Pacific/Kiritimati" -> 9131,
+      "Pacific/Kwajalein" -> 8632,
+      "MIT" -> 15338)
+    for (tz <- DateTimeTestUtils.ALL_TIMEZONES) {
+      DateTimeTestUtils.withDefaultTimeZone(tz) {
+        val skipped = skipped_days.getOrElse(tz.getID, Int.MinValue)
+        (-20000 to 20000).foreach { d =>
+          if (d != skipped) {
+            assert(millisToDays(daysToMillis(d)) === d,
+              s"Round trip of ${d} did not work in tz ${tz}")
+          }
+        }
+      }
+    }
+  }
+
   test("convert TZ with boundary time pacific") {
     val tz = TimeZone.getTimeZone("America/Los_Angeles")
     val boundaryDates = List(