If you have different clients who need to receive a different message on different days and different timestamps, you can best use the 'Batch import' function.
Create an excel file that contains multiple columns with the following values:
Originator - recipient - content of the message - scheduled date/time (Y-m-d H:i, e.g.: 2016-05-15 16:36)
This file can then be imported directly into your dashboard for scheduling.