Fix occasionally failing start_date specs

Pivotal Bug https://www.pivotaltracker.com/story/show/77245130.

Converted all uses of `Date.today` to `Date.current`. See [here](http://nikitaavvakumov.github.io/posts/date-in-ruby-vs-rails/) for explanation.
Seems like a violation of the Open/Close principle, if nothing else. I'd be tempted to change both models' attributes to simple `start_date` - let's see what @motasem-salem thinks.