The delivery of an SMS message may be delayed by setting an amount of time in minutes relative to the time at which it was received by our gateway. We will store the message until the required timeframe has elapsed. The maximum delay time is 10080 minutes or seven days. The actual delivery time of scheduled messages may be up to five minutes earlier than the scheduled time.
You can view the Parameters list page to see the name of the parameter for the API you are using (look at 'Delivery Time', not 'Scheduled Time').