[GitHub] [spark] MaxGekk commented on a change in pull request #27915: [WIP][SPARK-31159][SQL] Rebase date/timestamp from/to Julian calendar in parquet

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MaxGekk commented on a change in pull request #27915: [WIP][SPARK-31159][SQL] Rebase date/timestamp from/to Julian calendar in parquet
URL: https://github.com/apache/spark/pull/27915#discussion_r392945328
 
 

 ##########
 File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/DateTimeUtils.scala
 ##########
 @@ -974,4 +974,102 @@ object DateTimeUtils {
         }
     }.mkString("'")
   }
+
+  /**
+   * Converts the given microseconds to a local date-time in UTC time zone in Proleptic Gregorian
+   * calendar, interprets the result as a local date-time in Julian calendar in UTC time zone.
+   * And takes microseconds since the epoch from the Julian timestamp.
+   *
+   * @param micros The number of microseconds since the epoch '1970-01-01T00:00:00Z'.
+   * @return The rebased microseconds since the epoch in Julian calendar.
+   */
+  def rebaseGregorianToJulianMicros(micros: Long): Long = {
+    val ldt = microsToInstant(micros).atZone(ZoneId.systemDefault).toLocalDateTime
+    val utcCal = new Calendar.Builder()
+      // `gregory` is a hybrid calendar that supports both
+      // the Julian and Gregorian calendar systems
+      .setCalendarType("gregory")
+      .setDate(ldt.getYear, ldt.getMonthValue - 1, ldt.getDayOfMonth)
+      .setTimeOfDay(ldt.getHour, ldt.getMinute, ldt.getSecond)
+      .build()
+    millisToMicros(utcCal.getTimeInMillis) + ldt.get(ChronoField.MICRO_OF_SECOND)
+  }
+
+  /**
+   * Converts the given microseconds to a local date-time in UTC time zone in Julian calendar,
+   * interprets the result as a local date-time in Proleptic Gregorian calendar in UTC time zone.
+   * And takes microseconds since the epoch from the Gregorian timestamp.
+   *
+   * @param micros The number of microseconds since the epoch '1970-01-01T00:00:00Z'.
+   * @return The rebased microseconds since the epoch in Proleptic Gregorian calendar.
+   */
+  def rebaseJulianToGregorianMicros(micros: Long): Long = {
+    val utcCal = new Calendar.Builder()
 
 Review comment:
   It is set to the default system time zone. If we set it to to particular time zone, the conversion will be incorrect.
   
   Let me rename `utcCal` to `cal`.

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