%W is not handled properly

[joshuacronemeyer]

We see this:

irb(main):001:0> week = '201810'
=> "201810"
irb(main):002:0> Date.strptime(week, "%Y%W")
=> Mon, 05 Mar 2018
irb(main):003:0> require 'timecop'
=> true
irb(main):004:0> Date.strptime(week, "%Y%W")
=> Mon, 01 Jan 2018

Originally posted by @lisbethmarianne in #132 (comment)

[joshuacronemeyer]

I was closing #132 because the behavior referenced in the issue has changed and seems like not a bug anymore, but this comment was highlighting something that doesn't work. we don't handle the :week symbol that this date format causes in _strptime.