Ensure only valid claims can be completed

In investigating #87 I noticed that there was no validation that a claim existed and was valid when it was completed.

I have asked some questions in #90 to clarify what the intended behavior is, but I believe that it shouldn't be possible to complete a claim that is not currently valid.
What should we use for the start and end times for that claim? Should it just be `t` for start, end, and completed?