I think some of the autism-family/CDD disorders are characterized by a primary dysfunction of time estimation. We've used timers to countdown to the END of an activity, but this has limited success. Meltdowns are still very common.
On the other hand we've had very good success with countdowns to the START of an activity -- where failure to meet the countdown will result in LOSS of the activity. Of course this only works where:
1. The activity itself is desired OR
2. There is a reward associated with completion of the activity.
1. 3 minute countdown to bathtime (rewards is kookaid at bedtime -- so long teethies)
2. 3 minute countdown to getting out of house so can get siblings ready without fights/trauma (rewards is a swim outing).
In order to make this work, of course, one has to be quite ready to cancel the activity on failure to initiate it prior to the alarm. We've cancelled twice but succeeded more than 90% of the time.
When the setup can be well staged, and the subject knows the threat is not idle, this has been a proven technique. (Of course in this world one NEVER makes an idle threat.)