[amibroker] price at a certain date

[huandy631]

hi

I need to find out the return of a group of stocks over a certain 
period. The code below to find out the price at a certain date doesn't 
seem to work.

StartDate=DateTimeConvert(0,ParamDate("Start date","2007-06-
10"));                       
StartPrice=IIf(DateNum()==StartDate,ValueWhen(StartDate,Close),0);

Is this the right method?

thanks

Hugh