I'm testing a datepicker with rspec and capybara, some dates are disabled, and I think it would be easier to work with fixed test date.
Working with fixed dates breaks down into the server side and the browser side. For the server side you can use Timecop to set the date you want the server to think it is. On the browser side you can use something like Sinon.JS - There's an article here that talks about setting up your layout to automatically include sinon in tests where you're using timecop -- the article is from some time ago, but should be close enough.