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 based on week number.

Thanks.

Answer: 
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`.
[`Date#cweek`](http://ruby-doc.org/stdlib-2.2.2/libdoc/date/rdoc/Date.html#method-i-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"]

```