Because we live in a silly world where some people arbitrarily move their

clocks back and forth by an hour, timestamps, even those containing timezone

information, may not be unique. In particular, this could cause problems when

fetching the review-requests resource using the time-limiting fields with a

timestamp that exists during the hour after the transition from

daylight-savings time to standard time.
This change fixes things up to catch the error and return a reasonable error

via the API, rather than a 500 and a traceback. This also fixes it up so

invalid timestamp strings return an error rather than getting silently ignored.

Ran unit tests.