The named set are only computed once within a query. That is why your calculated member always return the same value.
You just have to remove the named set from your query:
MEMBER [Measures].[Test] AS {
StrToMember("[Time].[Date].&[" + [Project].[ParentProject].CURRENTMEMBER.PROPERTIES("Project Start Iso") + "]"):
StrToMember("[Time].[Date].&[" + [Project].[ParentProject].CURRENTMEMBER.PROPERTIES("Project End Iso") + "]")
}.COUNT