Hi Paul

There is quite a lot of work being dome on centralised notification in all areas including programs and certifications.

One of the improvements for this that is relevant is TL-9711  discussed in https://totara.community/mod/forum/discuss.php?d=18568

Using the current functionality have you tried setting a fixed due date for each cohort (audience) to the right month  and then using the program due message so the message is sent at different times for each cohort.

regards