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https://issues.apache.org/jira/browse/DERBY-4752?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12890803#action_12890803
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Knut Anders Hatlen commented on DERBY-4752:
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Instead of trying to fix the algorithm, I think it would be better to rely on
java.util.Calendar to do these calculations for us. That would require
allocating a Calendar object each time the method is called (in addition to all
the String objects the method already allocates), but it won't instantiate any
ResourceBundles or Locales, which was the reason why CheapDateFormatter was
written in the first place, according to the comments. I haven't found that
CheapDateFormatter is used in any performance critical section of the code, so
I'm not sure if that's much of a concern in any case.
> CheapDateFormatter returns incorrect and invalid date strings
> -------------------------------------------------------------
>
> Key: DERBY-4752
> URL: https://issues.apache.org/jira/browse/DERBY-4752
> Project: Derby
> Issue Type: Bug
> Components: Services
> Affects Versions: 10.7.0.0
> Reporter: Knut Anders Hatlen
> Priority: Minor
>
> CheapDateFormatter has multiple problems. These are the ones I'm aware of:
> 1) On the boundary between non-leap years and leap years it will return first
> day of thirteenth month in previous year (for instance, 2011-13-01 instead of
> 2012-01-01)
> 2) It treats all years divisible by four as leap years. Those divisible by
> 100 and not by 400 are not leap years. It attempts to adjust for that (see
> the snippet below) but it always ends up setting leapYear=true if (year%4)==0.
> // It's a leap year if divisible by 4, unless divisible by 100,
> // unless divisible by 400.
> if ((year % 4L) == 0) {
> if ((year % 100L) == 0) {
> if ((year % 400L) == 0) {
> leapYear = true;
> }
> }
> leapYear = true;
> }
> 3) More leap year trouble. To find out which year it is, it calculates the
> number of four year periods that have elapsed since 1970-01-01. A four year
> period is considered 365*3+366 days. Although most four year periods are of
> that length, some are shorter, so we'll get one day off starting from year
> 2100, two days off from year 2200, and so on.
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