Re: Date comparisons

From:
Nigel Wade <nmw@ion.le.ac.uk>
Newsgroups:
comp.lang.java.programmer
Date:
Fri, 12 Mar 2010 17:15:56 +0000 (UTC)
Message-ID:
<hndssc$mip$1@south.jnrs.ja.net>
On Fri, 12 Mar 2010 08:56:32 -0800, laredotornado wrote:

On Mar 12, 9:22??am, Thomas Pornin <por...@bolet.org> wrote:

According to laredotornado ??<laredotorn...@zipmail.com>:

I'm using Java 1.5. ??I have a java.util.Date object and I would like
to determine if it's date (i.e. year, month, and day) are greater
than (in the future) or equal to today's date (year, month, and day).
However, I don't care about any time component (hour, minute, second
...) when the comparison is taking place. ??What is the easiest way I
can determine this?


If you are in the UTC time zone (often called GMT too), then this is
simple. Use this:

?? ?? ?? ?? public static int dayCount(Date d)
?? ?? ?? ?? {
?? ?? ?? ?? ?? ?? ?? ?? return (int)(d.getTime() / 86400000L);
?? ?? ?? ?? }

which returns the date as an integral count of days since January 1st,
1970. You then just have to compare those day counts.

For other time zones, you will have to resort to Calendar and TimeZone.
Create a TimeZone instance for your time zone, then get a Calendar
instance (with Calendar.getInstance(TimeZone)), and use it to convert
your dates into years, months and days.

?? ?? ?? ?? --Thomas Pornin


Thanks. This is just the simple solution I was looking for. One follow
up . Why is the timezone important? If I know that both my Date objects
are the same time zone, wouldn't it still work even if that time zone
weren't GMT? - Dave


Because wall clock time (and this includes "what day is it" type
questions) in one timezone is different from wall clock time in another
timezone.

Consider the two instants in time 1 minute before midnight GMT and 1
minute after midnight GMT. They belong to different days in London, but
are both in the same day in New York (or any other timezone for that
matter). Your two Dates are in GMT, but you may well not be. Asking "what
day do these Dates represent" depends on what timezone you are in when
you ask the question.

Timezones are important. Ignore them at your peril.

--
Nigel Wade

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