Problem:

When I try to use the DAP Rollup action on a date field I get an error message. For example, when I use Due Date as the Rollup Field for Tasks, with Minimum as the Rollup Type, I get an error stating “field ActivityDate does not support aggregate operator MIN”.

Answer:

This is a limitation in Salesforce, where a number of Activity fields such as ActivityDate (e.g. Due Date in Tasks) cannot be used in roll-ups. 

As a workaround, create a custom date field under the same object, copy the values of the original date field into the custom field, and apply a roll-up to the custom date field.