Recently I heard another person complain that her spouse kept forgetting her birthday. I have long had problems with my partner forgetting commitments. I'm sure that my memory is imperfect as well.
It would be nice to have a perfect memory but, like a perfect body, we have to work with what we've got and there is no excuse for not doing what we can. And memory problems, like body problems, can be largely ameliorated with technical solutions.
The simplest solution is the humble pocket calendar. It's cheap, easy to carry around and if used habitually can solve the problem of forgetting joint time commitment.
However, today we have electronic calendaring solutions. These generate reminders that pop up on one's cellphone AND can be shared between the partners. There is absolutely no reason not to use them, unless one is paranoid about the NSA learning of your dental appointments and movie dates. Since they have already put a radio in the fillings of your teeth and are watching you through the movie screen, this is not an additional problem.
Working together as a couple to make this happen can actually be a caring response to a real problem, and turn into something fun! The key point is this: as with all relationship problems, the first and most important consideration is the will to solve the problem. Once that will is present, calendaring solutions can help.