Date field not working in Between operator

Hi,

I have a measure, say "Amount". I want a formula like this:

Total_amount=Sum([Amount] where ([Date] Between ([Week Start Date];[Week End Date]))

where [Week Start Date] and [Week End Date] are two Date variables having format "MM/dd/yy" which I got after applying ToDate function.

But I get an incorrect  result. What I get is the Total Amount, irrespective of the "Where" condition. It is as if the Where condition did not work at all.

However, when I place Constants ie.

Total_amount=Sum([Amount] where ([Date] Between ('5/1/14';'11/1/14'))

,the formula outputs correct results, ie. the Where condition is being applied appropriately.

Is there something I am doing wrong? Or is it that the Date fields need to be converted in a specific format?

Thanks,

Amrita