Quoteing svenbauhan / web.de, on Mon, Jan 31, 2005 at 06:10:47AM +0900:
> Hi,
> 
> I want to add a given time difference to an Time object. My first idea was
> to calculate the number of seconds of this time difference. This works for
> values like hours or days, but when I want to add a month, the number of
> seconds depends on the number of days in the given month.

Yep, not as simple as you might hope.

I extend Time in my vPim package to do this. Here's the code, note that
it uses Date (since Date knows about oddities of leap minutes, days in
a month, leap years, etc.). Note the rounding effects.

If you are doing more stuff with dates and times, you might find other
useful code in vPim.

Cheers,
Sam



=begin
  $Id: time.rb,v 1.4 2004/11/17 05:06:27 sam Exp $

  Copyright (C) 2005 Sam Roberts

  This library is free software; you can redistribute it and/or modify it
  under the same terms as the ruby language itself, see the file COPYING for
  details.
=end

require 'date'

# Extensions to builtin Time allowing addition to Time by multiples of other
# intervals than a second.

class Time
    # Returns a new Time, +years+ later than this time. Feb 29 of a
    # leap year will be rounded up to Mar 1 if the target date is not a leap
    # year.
    def plus_year(years)
      Time.local(year + years, month, day, hour, min, sec, usec)
    end

    # Returns a new Time, +months+ later than this time. The day will be
    # rounded down if it is not valid for that month.
    # 31 plus 1 month will be on Feb 28!
    def plus_month(months)
      d = Date.new(year, month, day)
      d >>= months
      Time.local(d.year, d.month, d.day, hour, min, sec, usec)
    end

    # Returns a new Time, +days+ later than this time.
    # Does this do as I expect over DST? What if the hour doesn't exist
    # in the next day, due to DST changes?
    def plus_day(days)
      d = Date.new(year, month, day)
      d += days
      Time.local(d.year, d.month, d.day, hour, min, sec, usec)
    end
end