Or you can do a Running Total with your specific date criteria to get one count and then a Summary count on the whole set to get the value:
{#RT}%COUNT(field)
This must be in the Report footer to show the correct value.
Or is you do not have duplicate records you can create a formula to SUM on, for example create a formula field called "DateCount' as:
If table.Datefield in BEginDate to ENdDAte then 1 else 0
Now you can SUM that and use it against the Record COUNT
This can be use in the report header.