The second phase in memorising a date is called the Revising Phase.
During this phase, TimeToTime.app will determine an optimal day on which to remind you to review a date, using a form of spaced repetition.
The purpose of this phase is to solidify the date in your memory. If you successfully guess a date during revision, you may see the same date again approximately 2 days, and then if you successfully guess it again, 5 days later and so on. This spaced repetition is what makes this form of memorising so effective.
All going well, a date will remain in the revising phase. You'll see it less and less often over time. TimeToTime does adjust the difficult of the questions for these more mature dates as well - trying to ensure you do remember the date more accurately.
Sometimes, things don't go well. Occasionally some dates are really tricky. When TimeToTime recognises that you are stuck on a date, it moves that date to the difficult queue - which you can address using the Difficult Phase - this is very similar to the Learning Phase where you'll be repeatedly exposed to a date over a short period of time, to reacquaint you with the date, and to give you an opportunity to create more links to help you memorise the date. After a date has exited the difficult queue, it will again be available in the Revising Phase.