Suggested by Ben Walker – Needs Votes
It seems that it is only possible to create Rollup Fields against Date-Time fields, not against Date-Only fields. I think it should be possible to create rollups on either type of Date field.
I can't find anything in the documentation that makes this clear either, so if this is not going to get fixed, maybe it should be added to the documentation as a limitation of Date-Only fields so people don't have to delete and recreate the fields when they realise this restriction. But I'd rather it got fixed... ;-)
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Sorry - this was a mistake on my part. Turns out that a.) You can only create a Date-only rollup on Date-Only fields, but also that the 'Behavior' attribute of the column must also be the same - I had one set to 'User local' and one to 'Date only' and so it wasn't allowing me to create the Rollup. This idea should be removed....