HI Elisa112,

Did you means the expression are used to calculate with static type values? If that is the case, you can try to use the following measure formula if it suitable for your requirement.

formula =
VAR intro =
    CALCULATE (
        MIN ( Table1[Date] ),
        FILTER ( ALLSELECTED ( Table1 ), [Type] = "Introduction" ),
        VALUES ( Table1[UserID] )
    )
VAR asse =
    CALCULATE (
        MAX ( Table1[Date] ),
        FILTER ( ALLSELECTED ( Table1 ), [Type] = "Assess" ),
        VALUES ( Table1[UserID] )
    )
RETURN
    DATEDIFF ( intro, asse, DAY )

If you mean these results will apply to calculate with dynamic rolling between different type of values, it should difficulty than calculate on specific values.

I'd like to suggest you to add an index filed of the type filed values, then you can use the current index to find out current type and previous type to get difference.

Regards,
Xiaoxin Sheng