Invalid numeric representation of date value: DATEDIFF

[Anonymous]

Hi 

I am trying to calculate 'Resolve Time' for incidents. I worked out the forumula, however the DAX formula is not returning the values for incidents which were open for a longer period of time. 

Resolve_Time = DATEDIFF(SUM('Call Request Detail'[OpenDate]), SUM('Call Request Detail'[ResolveDate]), DAY)

Below mentioned screenshot represents the  incidents which are opened and closed within 3 days.

 

However, if an incident is open for a longer period of time i.e. if an incident is opened on 25/11/2018 and closed on 06/12/2018. The calculation doesn't seem to be working. the follwing error pops out. 

FYI: I am using a direct connection to the DW, hence using the SUM function in my calculation. I understand that we can not access the source columns in DAX formulas when we use direct connection. 

Any advice will help me out. 

[Anonymous]

The SUM( ) of a date sounds... odd at best.  In the grand total you are *actually* adding dates, which... I'm surprised is even legal 😛 

I would do MIN() for Open and MAX( ) for Resolve... ?