I had implemented the computation of the latest timestamp of review entries

using datetime.ctime(), which in my sleep-deprived state I thought was

returning seconds since the epoch. In reality, it returns a string with a

human-readable date and time. This would often sort the way people expected

because in most cases, all the review entries come from the same month, and

other parts of the date string are increasing numerically, but it wasn't at all

correct.
I had done it this way because I was being lazy with my defaultdict. I've

changed things up to actually store and sort on the datetime objects directly

rather than trying to convert them to a number.

Ran unit tests.
Created a review request with a bunch of review entries and replies, and saw

  things work correctly.