Rails 3.2.8 - How do I get the week number from Rails?

Question

I would like to know how to get the current week number from Rails and how do I manipulate it:

1. Translate the week number into date.
2. Make an interval

Answer 1

Use strftime:

%U - Week number of the year. The week starts with Sunday. (00..53)
%W - Week number of the year. The week starts with Monday. (00..53)

Time.now.strftime("%U").to_i # 43

# Or...

Date.today.strftime("%U").to_i # 43

If you want to add 43 weeks (or days,years,minutes, etc...) to a date, you can use 43.weeks, provided by ActiveSupport:

irb(main):001:0> 43.weeks
=> 301 days

irb(main):002:0> Date.today + 43.weeks
=> Thu, 22 Aug 2013

irb(main):003:0> Date.today + 10.days
=> Sun, 04 Nov 2012

irb(main):004:0> Date.today + 1.years # or 1.year
=> Fri, 25 Oct 2013

irb(main):005:0> Date.today + 5.months
=> Mon, 25 Mar 2013

Answer 2

You are going to want to stay away from strftime("%U") and "%W".

Instead, use Date.cweek.

The problem is, if you ever want to take a week number and convert it to a date, strftime won't give you a value that you can pass back to Date.commercial.

Date.commercial expects a range of values that are 1 based. Date.strftime("%U|%W") returns a value that is 0 based. You would think you could just +1 it and it would be fine. The problem will hit you at the end of a year when there are 53 weeks. (Like what just happened...)

For example, let's look at the end of Dec 2015 and the results from your two options for getting a week number:

Date.parse("2015-12-31").strftime("%W") = 52
Date.parse("2015-12-31").cweek = 53

Now, let's look at converting that week number to a date...

Date.commercial(2015, 52, 1) = Mon, 21 Dec 2015
Date.commercial(2015, 53, 1) = Mon, 28 Dec 2015

If you blindly just +1 the value you pass to Date.commercial, you'll end up with an invalid date in other situations:

For example, December 2014:

Date.commercial(2014, 53, 1) = ArgumentError: invalid date

If you ever have to convert that week number back to a date, the only surefire way is to use Date.cweek.

Answer 3

Date#cweek seems to get the ISO-8601 week number (a Monday-based week) like %V in strftime (mentioned by @Robban in a comment).

For example, the Monday and the Sunday of the week I'm writing this:

[ Date.new(2015, 7, 13), Date.new(2015, 7, 19) ].map { |date|
  date.strftime("U: %U - W: %W - V: %V - cweek: #{date.cweek}")
}

# => ["U: 28 - W: 28 - V: 29 - cweek: 29", "U: 29 - W: 28 - V: 29 - cweek: 29"]

Answer 4

date.commercial([cwyear=-4712[, cweek=1[, cwday=1[, start=Date::ITALY]]]]) → date
Creates a date object denoting the given week date.

The week and the day of week should be a negative 
or a positive number (as a relative week/day from the end of year/week when negative). 
They should not be zero.

For the interval

require 'date'

  def week_dates( week_num )
    year = Time.now.year
    week_start = Date.commercial( year, week_num, 1 )
    week_end = Date.commercial( year, week_num, 7 )
    week_start.strftime( "%m/%d/%y" ) + ' - ' + week_end.strftime("%m/%d/%y" )
  end

  puts week_dates(22)

EG: Input (Week Number): 22

Output: 06/12/08 - 06/19/08

credit: Siep Korteling http://www.ruby-forum.com/topic/125140