[bug 1116498] Fix date filter API tests

This rewrites the date filter API tests with the following goals:

1. fixes the "hard-coded dates were older than 180 days from today"
   problem causing the tests to fail
2. cleans up the test code a bit to make it easier to read and
   reason about
3. removes a bunch of stuff that doesn't matter from the test setup
Merge pull request #427 from willkg/1116498-fix-tests

[bug 1116498] Fix date filter API tests