From: Yossef Mendelssohn <ymendel@...>
Date: 2007-09-26T23:37:02+09:00
Subject: Re: Finding the last Sunday of a month

On Sep 26, 9:10 am, Dale Martenson <dale.marten...@gmail.com> wrote:
> On Sep 26, 9:00 am, Yossef Mendelssohn <ymen...@pobox.com> wrote:
>
>
>
> > On Sep 26, 8:44 am, Peter Bailey <pbai...@bna.com> wrote:
>
> > > Hello,
> > > I need to find the date for the last Sunday in January, for any year. I
> > > need this for a budgetary script I'm trying to write. Using the
> > > Date/Time module, I've done this so far.
>
> > > require 'date'
> > > now = DateTime.now
> > > year = now.year
> > > d = Date.new(now.year, 1, 31)
> > > puts d.wday
>
> > > yields: 3
>
> > > This tells me that the last day of January is a Wednesday. But, I need
> > > that last Sunday. Is there a method in 'date' that can give me the date
> > > of the last Sunday? The last Sundays of each month are the boundaries
> > > for my company's budget periods.
>
> > > Thanks,
> > > Peter
> > > --
> > > Posted viahttp://www.ruby-forum.com/.
>
> > Not perfect, but something like this:
>
> > Cassady:~ yossef$ irb
> > irb(main):001:0> require 'date'
> > => true
> > irb(main):002:0> d = Date.new(2007, 2, 1)
> > => #<Date: 4908265/2,0,2299161>
> > irb(main):003:0> d.to_s
> > => "2007-02-01"
> > irb(main):004:0> d -= 1
> > => #<Date: 4908263/2,0,2299161>
> > irb(main):005:0> d.to_s
> > => "2007-01-31"
> > irb(main):006:0> d -= d.wday
> > => #<Date: 4908257/2,0,2299161>
> > irb(main):007:0> d.to_s
> > => "2007-01-28"
> > irb(main):008:0> exit
> > Cassady:~ yossef$ cal 1 2007
> >     January 2007
> >  S  M Tu  W Th  F  S
> >     1  2  3  4  5  6
> >  7  8  9 10 11 12 13
> > 14 15 16 17 18 19 20
> > 21 22 23 24 25 26 27
> > 28 29 30 31
>
> > The "secret", as I see it, is getting the last day of the month and
> > subtracting that date's wday from it.
>
> > Note that I start by getting the first day of the next month and going
> > back one day. I'm not sure about a sure-fire way to always get the
> > last day of a month by going forward (because you can get errors
> > trying to hit the 31st of September), but going back one day from the
> > first of the next month should work just fine.
>
> I think using -1 as the day of month gets you the last day of the
> month.
>
> irb(main):002:0> d = Date.new(2007, 1, -1)
> => #<Date: 4908263/2,0,2299161>
> irb(main):003:0> d.to_s
> => "2007-01-31"
> irb(main):004:0> (d - d.wday).to_s
> => "2007-01-28"
>
> --Dale

Hey, you're right!  At least that seems to work.

Thanks!

(Note: Getting this from you is apparently easier than reading the
documentation for Date.new (really Date.civil), which says "m and d
can be negative, in which case they count backwards from the end of
the year and the end of the month respectively.")

--
-yossef