I didn't really know where to put this, first I wanted to open a bug report but I wasn't sure that it is a bug.
when you open an event in the calendar, you'll get a possibility of assigning an a time to that event, then you can use a reminder to remind you 15 minutes before the event start.
The same principle is Present in tasks, but the one thing missing their is the possibility to define the time the task starts. This in terms prevents you from using the reminder feature in tasks, because it does not know when to remind you of the task.
sometime a task is performed in a certain time, but isn't actually an event.
It would be great if you can import this functionality from the calendar, that you can define a starting time for a task.