Dax format measure as date

I have a measure that finds the last date a certain action was done per client (before today). The measure does return the right date but I am not able to format it as a date. I need the measure to be an actual date because I want to be able to find the date diff between the measure date and TODAY().

My Measure:

Last  Call = 
VAR lastCheckIn = IF(CALCULATE(MAX(Activities[Due date]), 
    FILTER(ALLEXCEPT(Deals, Deals[Org Name]), Deals[Activities.Type] = "Check-in Call")
    )<>BLANK(),CALCULATE(MAX(Activities[Due date]), 
    FILTER(ALLEXCEPT(Deals, Deals[Org Name]), Deals[Activities.Type] = "Check-in Call"),
    USERELATIONSHIP(Dates[Date],Activities[Due date]),
    USERELATIONSHIP(Deals[Org Name],Activities[Org Name])


    ),"")
RETURN  

VAR oneBeforeLastCheckIn = IF(CALCULATE(MAX(Activities[Due date]), 
    FILTER(ALLEXCEPT(Deals, Deals[Org Name]), Deals[Activities.Type] = "Check-in Call")
    )<>BLANK(),CALCULATE(MAX(Activities[Due date]), 
    FILTER(ALLEXCEPT(Deals, Deals[Org Name]), Deals[Activities.Type] = "Check-in Call"),
    FILTER(Activities,Activities[Due date]<TODAY()),
    USERELATIONSHIP(Dates[Date],Activities[Due date]),
    USERELATIONSHIP(Deals[Org Name],Activities[Org Name])

    ),"")
  

return

VAR correctDate = IF(lastCheckIn>TODAY(),oneBeforeLastCheckIn,lastCheckIn)
RETURN

IF ( correctDate = BLANK(), 0, FORMAT(correctDate+ DATE ( 1899, 12, 30 ),"DD/MM/yyyy"))

All related date columns are formatted as date. Example of how my output does not work with DATEDIFF(). Strangely, many of the DATEDIFFs do work, see the yellow marked rows, but for some rows the outcome does not make sense. I just guessed this is because my Last Check in measure is not a date. (the "days since call" is a datediff between the above measure and today()). I tried formatting both like:

 FORMAT([Today],"YYYY/MM/DD")

but this did not work.