Because of the way the scheduler works, you cannot guarantee tasks will run at a specific time, but you can get pretty close.  Checkout this old blog I wrote back in 2006: Running on a Fixed Schedule.

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Joe Brinkman
DNN Corp.

Hi there,
I was wondering if there was an alternative to the DNN scheduler. I have some time sensitive tasks (gambling related) that need to run at various times throughout the day. The problem is that the times are not fixed. The frequency changes every minute.

In short, the closer to the event, the more frequent they need to run. The further away from the event, the less frequent they need to run. I have software that already knows the next time the schedule should run but my problem is that i cant seem to make DNN use my calculated nextstart or time-lapse frequency.

What i've done is at the end of any given task, i calculate the next time the task should run then set the TimeLapse frequency accordingly. However, it looks like DNN isn't using this value to determine the next start time for the schedule. Its using the TimeLapse value as it was before the task started. I can see that my value is committed to the DB but it seems too late at this point because DNN must be using the old values it read at the beginning of the task.

So any pointers on how i can better control the scheduler? Are there any alternatives out there?
Thanks
John