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From "Knut Anders Hatlen (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-4752) CheapDateFormatter returns incorrect and invalid date strings
Date Fri, 23 Jul 2010 08:35:50 GMT

    [ https://issues.apache.org/jira/browse/DERBY-4752?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12891520#action_12891520
] 

Knut Anders Hatlen commented on DERBY-4752:
-------------------------------------------

Out of curiosity, I changed the implementation of CheapDateFormatter.formatDate() to

    public static String formatDate(long time) {
        return new Date(time).toString();
    }

and to

    public static String formatDate(long time) {
        return new Date(time).toGMTString();
    }

The toString() variant (which localizes the timestamp) managed 390000 timestamps per second,
whereas the toGMTString() variant managed 500000.

So it seems like the performance of java.util.Date has improved a lot over these years and
it's now way faster than CheapDateFormatter. Perhaps now it's time to remove the CheapDateFormatter
class altogether and use java.util.Date instead?

> 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
>            Assignee: Knut Anders Hatlen
>            Priority: Minor
>             Fix For: 10.7.0.0
>
>         Attachments: CheapFormatterPerfTest.java, derby-4752-1a.diff, derby-4752-1b.diff,
derby-4752-1c.diff
>
>
> 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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