DHIS2 v2.30: Set Expiry Period Type and Expiry Days for Event Program

Hello, I’d like to set an expiry date for an event program in DHIS2 v2.30. However, I’m not clear how to do so based on the documentation.
Ideally, I would stop add/editing before a specific date (i.e. anything before 1-May-2021), but I think only relative dates are possible. So if I need to set a relative date I would like to set it up on a 7 month rolling basis, so if today is 1-Nov-2021, I wouldn’t want them to make any edits before 1-May-2021, then next month (1-Dec-2021) it would be 1-Jun-2021.

The example in the documentation says the following: “If you set the expiry type to “Monthly” and the expiry days to “10” and the month is October, then you can’t add or edit an event to October after the 10th of November.”
From the documentation of the date periods, the quarterly and six-monthly are a preset periods linked to specific months (i.e. quarterly: Jan-Mar, Apr-Jun) rather than a rolling basis.
If I did use the quarterly basis, could I set it to cover the last two quarters at least? How do the expiry days apply to quarters? If I put in ‘2’ does that cover the past two quarters or given the documentation example does it mean the 2nd day of the quarter?

What options are available to most closely reach my ideal goal for historical locking of the events?

Thanks,
Sarah