Unable to enter historical checkins once a current-day checkin has been recorded

[bklang]

As described. You can enter check-ins for days in the past as long as you have not entered one for the current day. This may be resolved by a fix to #434

Additionally, entering check-ins for previous days does not seem to complete correctly. The data is saved, but it's not clear that the check-in for the past day has occurred.