You are just going to have to write a bit of code that does the old "30 days hath September" thing on whatever they enter.
Not a particularly hard algorithm to construct...in fact now that I think about it...I think I may have something.
(browses hard drive)
here is a JS function you can use as a start. If you wanted to get REAL flash, you could check to see if this year is a leap year before deciding what to do with February...
//===========================================================
// validate date
//===========================================================
function validateDate(field) {
var input = parseInt(field.value, 10)
if (isEmpty(input)) {
alert("Be sure to enter a date value."

select(field)
return false
} else {
if (isNaN(input)) {
alert("Entries must be numbers only."

select(field)
return false
} else {
var monthField = document.addUserDetails.month
if (!validateMonth(monthField, true)) return false
var monthVal = parseInt(monthField.value, 10)
var monthMax = new Array(31,31,29,31,30,31,30,31,31,30,31,30,31)
var top = monthMax[monthVal]
if (!inRange(input,1,top)) {
alert("Enter a number between 1 and " + top + "."

select(field)
return false
}
}
}
calcDate()
return true
}
function validateMonth(field, bypassUpdate) {
var input = parseInt(field.value, 10)
if (isEmpty(input)) {
alert("Be sure to enter a month value."

select(field)
return false
} else {
if (isNaN(input)) {
alert("Entries must be numbers only."

select(field)
return false
} else {
if (!inRange(input,1,12)) {
alert("Enter a number between 1 (January) and 12 (December)."

select(field)
return false
}
}
}
if (!bypassUpdate) {
calcDate()
}
return true
}
function validateYear(field) {
var input = parseInt(field.value, 10)
if (isEmpty(input)) {
alert("Be sure to enter a month value."

select(field)
return false
} else {
if (isNaN(input)) {
alert("Entries must be numbers only."

select(field)
return false
} else {
if (!inRange(input,1900,2005)) {
alert("Enter a number between 1900 and 2005."

select(field)
return false
}
}
}
calcDate()
return true
}
=====================
Good luck
Steve Davis
hey.you@hahaha.com.au