Rollups...are they finnicky?

This is mainly an attempt to discover why my rollup columns recognize system generated columns, but not user generated columns.

I have a form that allows users to record transactions, and specify the date of the transaction, but when I try to create a rollup that references that column, it doesn't appear as an option, but the two system generated date/time columns, CreatedOn and ModifiedOn do show up....any thoughts?