Our new MD1 update brings custom reminder times to your events.
When you create a new event you can choose to add a custom reminder time. Tapping on custom, will lead you to the date and time picker, where you can then pick the exact moment you want the reminder to trigger. This can now even be set to occur after the event has started or ended. (Though it might not work with all calendar servers so this is still experimental.) If you'd like to set a default reminder that automatically accompanies each new event, you can do this by going to DigiCal > menu > preferences > events > default reminder time. Though the ability to set a custom default reminder time isn't available yet.