Calculating a day given a Date

Question

From a given Date I need to calculate midnight of its day. Here's what I came up with. It's so ugly that I figure there must be a better way.

private Date day(Date creation) {
    Calendar calendar = Calendar.getInstance();
    calendar.setTime(creation);
    calendar.set(Calendar.HOUR_OF_DAY, 0);
    calendar.set(Calendar.MINUTE, 0);
    calendar.set(Calendar.SECOND, 0);
    calendar.set(Calendar.MILLISECOND, 0);
    return calendar.getTime();
}

Suggestions?

Kent

Answer 1

You should consider the built-in date API obselete. Instead use the Joda date and time API.

Here's a drop-in replacement for your method.

private Date day(Date creation) {
    return new DateMidnight(creation).toDate();
}

Here's a simple test:

public static void main(String[] args) {
    final Date creation = new Date();
    final Date midnight = new Foobar().day(creation);
    System.out.println("creation = " + creation);
    System.out.println("midnight = " + midnight);
}

The output is:

creation = Sun May 31 10:09:38 CEST 2009    
midnight = Sun May 31 00:00:00 CEST 2009

Answer 2

JODA might have a better solution, at the cost of another dependency on a library. I'm looking at its DateMidnight property. I'm sorry that I can't answer more authoritatively, but I'm not a JODA user myself.

Answer 3

If you're looking for local time, that's the best you're going to get. And although you may consider it ugly, there's (potentially) a lot going on behind the scenes.

If you want midnight UTC, the following will work:

public static void main(String[] argv)
throws Exception
{
    final long MILLIS_PER_DAY = 24 * 3600 * 1000L;

    long midnightUTC = (System.currentTimeMillis() / MILLIS_PER_DAY) * MILLIS_PER_DAY;
}

Edit: I really don't recommend using local dates in "production" code. They cause more trouble than they're worth -- consider a person on the US west coast who suddenly finds his/her "day" trimmed by 3 hours because an east coast computer has a different idea of midnight.

Answer 4

JODA is the way to go if you have serious calendar needs, at least until JSR-310 gets into the JDK (1.7 maybe, if not 1.8).

That being said, there are a couple of things that could be done to make this code a little nicer.

import static java.util.Calendar.*;

...

private static final List<Integer> TIME_FIELDS = 
    Arrays.asList(HOUR_OF_DAY, MINUTE, SECOND, MILLISECOND);

private Date day(Date creation) {
     Calendar c = getInstance();
     c.setTime(creation);
     for(int field : TIME_FIELDS) c.set(field, 0);
     return c.getTime();
}

That won't win any performance awards. You could do a standard for loop relying on the specific field values (the Calendar class has a FIELD_COUNT field kind of implying you can do something like that) but that risks issues across JDK implementations and between versions.

Answer 5

Using the date4j library :

DateTime start = dt.getStartOfDay();