How can I add a reminder to a calendar event at a specific date and time? On my Samsung Galaxy S2, I can't add a useful reminder time. The calendar only offers 5 minutes, 10 minutes, ... one day, two days, ...

This is utterly useless. What I usually need is "19:15 the day before the event" or remind me 12:00 on the 13th.

How do I do that?
Create a new appointment and then click the green + sign under the REMINDERS section. At the very bottom of the list there's a CUSTOMISE option that allows you to specify the number of minutes / hours that you want the reminder to occur.

Certainly not as good as a fixed time occurrence but it certainly gives you a close workaround.