viirya commented on a change in pull request #23000: [SPARK-26002][SQL] Fix day
of year calculation for Julian calendar days
URL: https://github.com/apache/spark/pull/23000#discussion_r242411040
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File path:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/DateTimeUtils.scala
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@@ -53,14 +53,27 @@ object DateTimeUtils {
final val MILLIS_PER_DAY = SECONDS_PER_DAY * 1000L
- // number of days in 400 years
+ // number of days in 400 years by Gregorian calendar
final val daysIn400Years: Int = 146097
+
+ // In Julian calendar every year which is a multiple of 4 is a leap year.
+ // So in 400 years there are 3 extra leap years compared to the Gregorian
calendar:
+ // 100, 200 and 300 (as multiples of 100 but not 400).
Review comment:
Actually I must read
https://en.wikipedia.org/wiki/Julian_calendar#Year_length;_leap_years to
understand why there are three extra days here. I'm not sure if this added
comments are enough clear to others.
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