The only time based items for automatic groups would be based on an additional field which would need to be set to a date somehow and then you have the choices are on, is before, is after, is in range, so if the timing is consistent you could do is before or is in range of, but I suspect the group needs to handle overlapping timeframes? If not then you should be able to do with the above, if it is not then you need some type of external process most likely that checks for users that fit criteria and removes them from the group via API. 

So we are saying that in a Docebo, is is impossible with an out-of-the-box functionality (automated groups) to enroll people

• in set of courses A after 1 month,
• set of courses B after 3 months
• … and all of that regardless of their hiring date

Looks like a very basic need for me

I believe most people get around this using additional fields and setting them to the date that triggers the enrollment, but that requires some logic/math to be done somewhere and update the field, not sure if the process you are using to add the users initially could support that?