Go to the host menu and then the SQL menu
Run the following script
SELECT *
FROM dnn_Schedule
Then you get the id of the schedule you need to modify.
Now you run the following script to get the id of the record we need to modify
SELECT ScheduleHistoryID, ScheduleID, StartDate, EndDate, Succeeded, LogNotes, NextStart, Server
FROM dnn_ScheduleHistory
WHERE ScheduleID = [Replace this with your scheduleID]
ORDER BY ScheduleHistoryID DESC
Now that we have this information you can run the following update script
UPDATE dnn_ScheduleHistory SET NextStart = [The date that it needs to be executed]
WHERE ScheduleHistoryID = [Your historyID]
You have to note that if your application restarts this can change things again. So there is no actual way of making a job at a specified time.
At my previous job I did this for a customer by running a stored procedure in the application start that would change the NextStart of the stored procedure.