Hi @Speedydgon ,
You can take an idea from here on how to create the measure you are looking for:
To get the prior date you need to add -2 in the formula:
ChildrenSet([Fix Version].[Ordered release dates]).Item(
Rank(
[Fix Version].CurrentMember,
ChildrenSet([Fix Version].[Ordered release dates])
)-2
).get('Release date')
In the report it looks like this:
best,
Gerda // support@eazyBI.com