From: Sam Roberts <sroberts@...>
Date: 2004-03-21T05:54:55+09:00
Subject: Re: bug in Time, it wraps some dates to next month

Quoteing hal9000@hypermetrics.com, on Sun, Mar 21, 2004 at 05:20:09AM +0900:
> Sam Roberts wrote:
> >Possibly you are thining of Date.new(), which allow negative months, and
> >negative daysofmonths, like the C mktime()? However, it doesn't allow
> >out of range values:
> >
> >[ensemble] ~/p/ruby/vcard $ ruby -rdate -e "p Date.new(2004, 2, 29).to_s"
> >"2004-02-29"
> >[ensemble] ~/p/ruby/vcard $ ruby -rdate -e "p Date.new(2004, 2, 30).to_s"
> >/usr/lib/ruby/1.6/date.rb:147:in `new3': invalid date (ArgumentError)
> >        from -e:1
> 
> Personally I think I'd like to reduce our dependence on the C lib.

In terms of freedom from idiosyncracies of the systems C library
implementation, I absolutely agree.

> I think I'd like to see an exception or at least a nil return value
> for (e.g.) Feb 29, 2005 or November 31 of any year.

I could live with that, though what would be more useful is for this to
work:

[ensemble] ~/p/ruby/vcard $ irb
irb(main):002:0> now = Time.now.to_a
[44, 30, 15, 20, 3, 2004, 6, 80, false, "EST"]
irb(main):003:0> now[3] += 40
60
irb(main):004:0> now
[44, 30, 15, 60, 3, 2004, 6, 80, false, "EST"]
irb(main):005:0> later = Time.mktime(*now)
ArgumentError: argument out of range
  from (irb):5:in `mktime'
  from (irb):5

I would like to see this give me 3:15PM on the day 40 days from the
current day.

Right now, a naive implementation would be to take Time.now, and add 40
times the number of seconds in a day.

But, if you went past a DST change, that wouldn't give you 3:15PM!

Instead, I do this:

class Time
  # Addition of a single day, respecting the local timezone.
  # 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

Because Date knows how to have days added to it, and to wrap past the
end of a month.

Anybody have a better suggestion? Perhaps I'm missing something, or
maybe 1.8 has a better way?

It's quite difficult to correctly do date manipulations in the face of
timezones and DST, and ruby has some powerful date libraries, but they
are very low-level, doing things like adding and subtracting units of
months/days/weeks from a Time is not easy.

I've some methods to propose as standard, or just on the wiki, for doing
this kind of thing. Maybe tomorrow they will be finished.

> I wonder if this would break any existing code? Probably something

Wouldn't be the only thing in 1.8 to break existing code!

Cheers,
Sam