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https://issues.apache.org/jira/browse/HIVE-1253?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12846669#action_12846669
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mingran wang commented on HIVE-1253:
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To get to the bottom of this, the formatter (SimpleDateFormat) interprets the
time string as local time zone, when it sets the calendar, it is PDT time zone
(7 hours difference from UTC), and when it substracts 7 days and convert back
to formatter string, it converts UTC to PST (still the local time zone, but
different time zone now, and 8 hours difference from UTC).
For the purpose of this function, since it is doing simple date_sub function,
there is no need to consider time zone at all, and all time zone should give
the same result for this kind of computation. For this reason, using default
time zone would be fine, and we have to use default time zone because formatter
is using it.
> date_sub() function returns wrong date because of daylight saving time
> difference
> ---------------------------------------------------------------------------------
>
> Key: HIVE-1253
> URL: https://issues.apache.org/jira/browse/HIVE-1253
> Project: Hadoop Hive
> Issue Type: Bug
> Components: Query Processor
> Affects Versions: 0.5.0
> Reporter: mingran wang
>
> date_sub('2010-03-15', 7) returns '2010-03-07'. This is because we have time
> shifts on 2010-03-14 for daylight saving time.
> Looking at ql/src/java/org/apache/hadoop/hive/ql/udf/UDFDateSub.java, it is
> getting a calendar instance in UTC time zone.
> def calendar = Calendar.getInstance(TimeZone.getTimeZone("UTC"));
> And use calendar.add() to substract 7 days, then conver the time to
> 'yyyy-MM-dd' format.
> If it simply uses default timezone, the problem is solved: "def calendar =
> Calendar.getInstance());"
> When people use date_sub('2010-03-15', 7), I think they mean substract 7
> days, instead of substracting 7*24 hours. So it should be an easy fix. The
> same changes should go to date_add and date_diff
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