I see that you already made two variables, vStartDate and vEndDate, So you can manipulate them using a Calendar or an Input Box.
In this case your set expression is almost good. It sould look like this:
Sum({$<Date = {">=$(=vStartDate)<=$(=vEndDate)"}>} Target)
So in the curly bracket it starts with a $ sign, which represents the current selection, which will be filtered by the expression. In the double quotes you tell that the value of Date should be bigger or equal than the evaluated =vStartDate
expression (evaluation is done by using a dollar-sign expression), and less than vEndDate. This is how it works.