Issue with Date Picklist

I have a form with a list of valid dates.
I have another form where the valid dates are referred to as a picklist.

I want to limit the number of records entered with a particular date, but I have a problem when I try and use the input.Date_field in my form validation. I can access the field fine in the field action, but I need to do it from the validation in order to cancel submit.

The code is:
on add
{
on validate
{
if (count(User_Status[Date_field == input.Date_field]) > 5)
{
alert "Only 5 records allowed for a given date";
cancel submit;
}
}
}

The same code works fine if I use another field in the validate, but even mentioning the input.Date_field seems to cause my form to stop working.
Is there an issue referring to Date picklists in the 'on validate' ?

my application is:
creator.zoho.com/holgum/form/8