Asp.net

I am designing a website which uses dates input to calculate the total cost of product, using this code

pdatstart =request.form(txt1)
pdatend=request.form(txt2)
plngwd=datediff(dateinterval.day,pdatstart,pdatend)

but it returns an error saying cast from string to type date is not valid
and how can i use it to calculate for the cost in the shopping basket.

ThanxxAre you using this in Crystal Report formula field?