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https://issues.apache.org/jira/browse/SPARK-29328?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Sean R. Owen resolved SPARK-29328.
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Fix Version/s: 3.0.0
Resolution: Fixed
Issue resolved by pull request 25998
[https://github.com/apache/spark/pull/25998]
> Incorrect calculation mean seconds per month
> --------------------------------------------
>
> Key: SPARK-29328
> URL: https://issues.apache.org/jira/browse/SPARK-29328
> Project: Spark
> Issue Type: Bug
> Components: SQL
> Affects Versions: 2.1.3, 2.2.3, 2.3.4, 2.4.4
> Reporter: Maxim Gekk
> Assignee: Maxim Gekk
> Priority: Minor
> Labels: correctness
> Fix For: 3.0.0
>
>
> Existing implementation assumes 31 days per month or 372 days per year which
> is far away from the correct number. Spark uses the proleptic Gregorian
> calendar by default SPARK-26651 in which the average year is 365.2425 days
> long: https://en.wikipedia.org/wiki/Gregorian_calendar . Need to fix
> calculation in 3 places at least:
> - GroupStateImpl.scala:167: val millisPerMonth =
> TimeUnit.MICROSECONDS.toMillis(CalendarInterval.MICROS_PER_DAY) * 31
> - EventTimeWatermark.scala:32: val millisPerMonth =
> TimeUnit.MICROSECONDS.toMillis(CalendarInterval.MICROS_PER_DAY) * 31
> - DateTimeUtils.scala:610: val secondsInMonth = DAYS.toSeconds(31)
> *BEFORE*
> {code}
> spark-sql> select months_between('2019-09-15', '1970-01-01');
> 596.4516129
> {code}
> *AFTER*
> {code}
> spark-sql> select months_between('2019-09-15', '1970-01-01');
> 596.45996838
> {code}
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