in FMP10, you filter portals by manipulating the relationship between Table Occurrences.
If you are already related by SerialID, add a field to your parent TO which is global and can be switched from "working hours" to "after hours".
Then adjust your relationship between Parent and Child TO's to be
Parent::SerialID = Child::SerialID
AND Parent::globalHourType = Child::HourType
Now by changing the value of the Parent::globalHourType, you affect which records are related to the parent record, thus changing the records showing up in the portal...or "filtering" it.
The same can be done with date range by having Parent::globalStartDate and Parent::globalEndDate and adding
AND Parent::globalStartDate < Child:: Date
AND Parent::globalEndDate > Child:: Date
to your relationship definition.
This is what you're looking for. Did I guess your proficiency level well enough?