Choose "DateVariableField" for data type
"RangeValue" for Range Type.
Then you will have 2 fields to enter variable dates.
Example 1st of last month, last of last month:
{DATEADD(Months|{DATEFORMAT(yyyy-MM-01)}|yyyy-MM-dd|-1|yyyy-MM-dd)}
{DATEADD(Days|{DATEFORMAT(yyyy-MM-01)}|yyyy-MM-dd|-1|yyyy-MM-dd)}