Groovy DateTime Math

DateTime is capable of simple increment (++), decrement (--), and all the add/roll methods provided by Calendar. Here is an example of some of the ops:

refDate = new Date()
dt = new DateTime(date:refDate)
tomorrow = dt + 1
yesterday = dt - 1

assert refDate == dt // insure that the original did not change
assert refDate + 1 == tomorrow
assert refDate - 1 == yesterday
assert refDate + 7 == nextWeek

Groovy Equality: Groovy evaluates '==' as equality, not identity. So it was necessary to override equals() to compare to the millisecond and return true or false. The override makes it possible to return true when the underlying milliseconds match whether from Date, Calendar, Long, or DateTime.

Problems with Milliseconds: When working through the plus and minus methods I made the mistake of using the raw milliseconds to add days. This worked fine on my linux machine, but mysteriously died on OSX. What I neglected to take into consideration was the switch between standard and daylight time (luckily my development cycle is close to the change or I would have missed it). So, a quick fix was to rely on java.util.Date to add days and return a new DateTime copy. A better approach may be to use groovy's Duration classes...

Durations: The groovy library includes a set of duration classes in the groovy.time package. The duration classes are containers for years, months, days, hours, minutes, seconds and millis. They support basic math functions to add and subtract durations--very groovy. DateTime uses the duration classes for basic math with the following methods:

now = new DateTime()
current = now.asDuration()
twodays = new Duration(2, 0, 0, 0, 0)
dt = DateTime.fromDuration( current + todays )

Combined with groovy's TimeCategory, you can do this:

use(TimeCategory) {
  future = new DateTime(date:2.weeks.from.now)
  oneWeek = 1.week  // this is a duration

  // schedule something once each week for the next 5 weeks...
  dates = []
  5.times { dates << it.weeks.from.now }
}

Additional utility methods and parsing to come next...