Re: Date comparisons

From:
Patricia Shanahan <pats@acm.org>
Newsgroups:
comp.lang.java.programmer
Date:
Fri, 12 Mar 2010 09:21:18 -0800
Message-ID:
<MvCdnYys8aiP5QfWnZ2dnUVZ_hWdnZ2d@earthlink.com>
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


It is a matter of setting appropriate boundaries between yesterday,
today, and tomorrow. I'm writing this in California, Pacific Standard
Time, in the morning. An event 10 hours ago happened late yesterday,
from my point of view. Now consider someone in London. For them, it is
now early evening and an event 10 hours ago happened this morning.

Patricia

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