When giving the option for something to reoccur every certain amount of time how should I treat times that don't reoccur on every interval?
For example what should happen to birthday reminders for February 29th? Or if I have a monthly appointment on the 31st what should happen on months that do not have a 31st day?
What do you believe the reasonable user would expect and be least surprised by?
My first reaction to this question would be to give the user the option of what to do if it falls on that date (go to next day, skip the day, etc.).
But to directly answer your question, I believe the reasonable user would be least surprised by the occurrence falling on the day before the "skipped" day.
I'd think you would be able to flag dates like this pretty easily. The only dates I can think of are the 29th, 30th, and 31st of a month, or February 29th.
When the user opts to be reminded of one of these dates monthly (or annually for Feb. 29), you can prompt them for an alternate date for those months.
Additionally, you could have an option to be notified "on the last day of each month."
Ideally, prompt when creating the reminder.
If you are stuck with it, I would pull them forward. That's the least harmful choice. You don't always want the least surprise, minimizing the harm of making the wrong choice is also important.