How can i calculate different date of transition status of two status?

[amiriamiri]

DateDiffDays(
CASE WHEN [Transition Status].[To Do] > 0 then
TimestampToDate(
[Measures].[Transition to status last timestamp]
),CASE WHEN [Transition Status].[Done] > 0 then
TimestampToDate(
[Measures].[Transition to status last timestamp]
))

I try another query but not work

DateDiffDays(([Measures].[Transition to status last date],[Transition Status].[To Do]),
([Measures].[Transition to status last date],[Transition Status].[Done]))