Pivot Table Data Model Using Difference From

[excelbytes]

I have a pivot table with clients ("Client") and dates visited ("Date"). In the Rows I have Client and below that Date. In the Values field I have Date and formatted it to be Show Values As - Difference From - [previous]. Works fine.

However, doing the same set up when the same table is part of a Data Model, it won't calculate the difference from the previous date. Why not and is there a way around this?