From: Yossef Mendelssohn <ymendel@...>
Date: 2010-02-10T03:58:28+09:00
Subject: Re: algorithm to truncate date to beginning of week?

On Feb 9, 12:32 pm, Chuck Remes <cremes.devl...@mac.com> wrote:
> My end goal is to always be able to pick the first Sunday at midnight backwards from a given date. If there is another approach to accomplish this, please share.

Use the logic inherent in Date objects. They can tell you the weekday
(as an integer, 0-based starting at Sunday), and subtraction/addition
work on days.

    $ irb -rdate
    >> d = Date.today
    => #<Date: 4910473/2,0,2299161>
    >> d.to_s
    => "2010-02-09"
    >> d-d.wday
    => #<Date: 4910469/2,0,2299161>
    >> _.to_s
    => "2010-02-07"

A Date object is just that, a date. If you mean a time, you can
recreate the date using parts of the time.

    >> t = Time.now
    => Tue Feb 09 12:54:48 -0600 2010
    >> d = Date.new(t.year, t.month, t.day)
    => #<Date: 4910473/2,0,2299161>
    >> d.to_s
    => "2010-02-09"

Or if you still need a Time object when you're done, you can go the
other way as well.

    >> t = Time.local(d.year, d.month, d.day)
    => Tue Feb 09 00:00:00 -0600 2010


HTH HAND

--
-yossef