Hi Cheryl .- I will have to research why this is happening, its possible that its trying to cast and verify its a date and since its blank its throwing the error but this is used in a few places so not sure yet where the actual error is.
My suggestion is to use a textbox for the date as you have more flexibility for regular expressions and actually even better date fields. Please check out the date field at
http://www.datasprings.com/formdemos, I think specifically the jQuery date fields on demo 28 and see if that can at least provide you with a work around (probably better then a work around).
Thanks,
Chad