Martin at TipMonkies has looked into the aspect of how to manage time-specific events in our lives in term of GTD. He talked about it is flawed if one does not have alarm facility in their GTD system:
… I record the information into a device and set an alarm so that it goes off at the point when I must take action (leave for the appointment), maybe even 15 minutes before I need to leave. October 1 rolls around. I proceed through the day, oblivious. If I do happen to notice ahead of time that I have a 3PM commitment, I am at rest with it as I see it is alarmed and taken care of. There is nothing to concern myself with. The system is in place. 2:30 rolls around, my device sounds an alarm. Curious, I check it. “Oh yeah. Dentist.” I leave…
Yes, to able to clear the task out of your mind and care free, you really need to have alarm to remind you the time-specific tasks.
Intrusive Alarms and GTD – [TipMonkies]