I'm not sure if this is the easiest, but it works.
You can create new columns.
expr2: iif([DATEFIELD1] is null,1,0)
expr3: iif([DATEFIELD2] is null,1,0)
These columns will have a 1 in them if the [DATEFIELD] is null, or a 0 if not.
You can then count the 1s and that will give you a count of...