use Timecop to make EventController spec resilient to the passage of time

Some notes:
* I was expecting to find and address more cases where the specs were date dependent, but this group of tests are the only ones, as far as I can tell.
* As a bonus, we are also no longer coupling the tests to a private controller method, which is positive.
* I don't love the heavy usage of metaprogramming in these specs, but that concern seems out of scope for this PR.