The simplest type of proactive reminder system is essentially an alarm clock, acting to acquire the user's attention and delivering reminders at previously specified times. Most electronic organizers have an alarm-clock feature, in which reminders may be scheduled for delivery at specified times. When the appointed time arrives, a general audible or vibration alert is produced to attract the user's attention; it is then up to the user to acknowledge the alert and receive the specifics of the reminder.
Time based proactive reminder systems are simple, reliable, and limited. Reminders cannot be conditioned on location or the actions of the user. It is only for tasks in which location and action are fixed or do not matter that such reminders work well; the classic example is the bed-side alarm clock which works reliably to alert a user of known location (the bed) and action (sleeping) that it is time to get up. (The user may choose to ignore this reminder, of course, or reschedule through interacting with the "snooze" button.)