Date columns in IF function

Hi!

I have two date columns:

•dim_leads[meeting_date]
•dim_leads[closed_date]

What I want is to:

if dim_leads[closed_date] = BLANK(), 

then calculate the number of leads according to dim_leads[meeting_date], otherwise according to dim_leads[closed_date]. 

I've tried this:

IF(

    dim_leads[closed_date] = BLANK(),

    CALCULATE(

               DISTINCTCOUNT(dim_leads[id]),

               USERELATIONSHIP(dim_date[Date],      dim_leads[meeting_date])

,

    CALCULATE(

               DISTINCTCOUNT(dim_leads[id]),

               USERELATIONSHIP(dim_date[Date],      dim_leads[closed_date])

)

... but the thing is that it returns me the error because IF can not define a single value from date column and requires me to specify an aggregate which is not what i'm looking for. 

Can you help me with this, please? How can I combine IF function with date column in it?