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.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>