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

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.