IMHO... you nneed to look at the regular expression, try here for an idea http://www.regular-expressions.info/dates.html
also, your form
<input type="text" id="fbdy" name="fbdy"/>
<input type="submit" value="Register" onclick="return Validatedate()" />
has no name, action or method and should be IMHO
<input name="fbdy" type="text" value="" />
<input name="submit" type="submit" value="Register" />
and pass the reference to the function via the 'this' object and in addition you should always have three elements to an input field, name, type and value tags even if the value of the field is empty, you should have them tags.
Your script is them simplified to...
var chkdate = o.fbdy.value || "";
the variable chkdate will = the value of the field fbdy or an empty string and the whole form will be available through the object this represented by the variable o in the function which means that if you add more input fields, they are accessible via o.formName.value
When your form validation works, you can then decide on the forms action.