It is not possible for end users to set maintenance windows retroactively through the UI. Please contact CA Support, and supply the following information:
1. That retroactive maintenance is needed
2. The email address of your primary/master account
3. The name of the monitor(s)
4. The data and time windows (eg Jan 1 2017 from 10 AM to 12 PM EST) for all retroactive maintenance.
CA Support will then make changes to the ASM backend database to retroactively set the monitors to be in maintenance during those periods.