How to do calendar operations in Java GWT? How to add days to a Date?

Question

Since GWT does not provide the GregorianCalendar class, how to do calendar operations on the client?

I have a Date a and I want the Date, which is n days after a.

Examples:

a (2000-01-01) + n (1) -> 2000-01-02
a (2000-01-01) + n (31) -> 2000-02-01

Solution 1

Updated answer for GWT 2.1

final Date dueDate = new Date();
CalendarUtil.addDaysToDate(dueDate, 21);

Edit: the fully qualified name of this class is com.google.gwt.user.datepicker.client.CalendarUtil.

Solution 2

The answer that Google seems to use (currently), is:

  @SuppressWarnings("deprecation") // GWT requires Date
  public static void addDaysToDate(Date date, int days) {
    date.setDate(date.getDate() + days);
  }

This is from the class com.google.gwt.user.datepicker.client.CalendarUtil, which is used by com.google.gwt.user.datepicker.client.DatePicker. I imagine, that there will be problems involved, when doing calculations in different timezones.

Lots of people have already voted for some kind of Joda time for GWT: http://code.google.com/p/google-web-toolkit/issues/detail?id=603 . The currently last comment states, that there's a new fork of goda time, maybe we should really check it out.

Solution 3

private static final long MILLISECONDS_IN_SECOND = 1000l;
private static final long SECONDS_IN_MINUTE = 60l;
private static final long MINUTES_IN_HOUR = 60l;
private static final long HOURS_IN_DAY = 24l;
private static final long MILLISECONDS_IN_DAY = MILLISECONDS_IN_SECOND *
        SECONDS_IN_MINUTE *
        MINUTES_IN_HOUR *
        HOURS_IN_DAY;
public Date addDays (Date date, days)
{
 return new Date (date.getTime () + (days * MILLISECONDS_IN_DAY));
}

this will work with leap years but will eventually stray by milliseconds on milleniums when we add or drop leap seconds.

Solution 4

I've created a rough implementation that emulates TimeZone, Calendar, and Locale. Feel free to try it out here:

http://code.google.com/p/gwt-calendar-class/downloads/list

View more solutions

Comments

[Chris Lercher]

+1, but please note: It depends very much on what you want to achieve. Usually, adding 7 days to "2011-03-22 10:00:00 CET" should result in "2011-03-29 10:00:00 CET" (i.e. the local time should stay at 10 o'clock). But because in CET, a clock change (DST) occurs between the two dates, the millisecond calculation will yield a different result. Again, it depends on what kind of calculation you want!

[cellepo]

The alternative answer by @Chris Lercher can help if you need to add a unit of time besides Days (i.e: hours)

[cellepo]

You can similarly call Date's other set methods for adding different units of time (i.e: setHours)

[Alex K.]

This fails for daylight saving. Over a daylight savings night, adding 24 hours is not the same as adding 1 day.