With your help, here's what I came up with (and it works!):
<!--Validate that the length of the birth year field is 4 and all numbers-->
Zip Code (5 Numbers):   <input name="zip" type="text" value="" onchange="if((this.value.length != 4) || (isNaN(this.value))) alert('Please enter your Birth Year.')">
I'm trying to do as much "in-line" (actually on the text fields using "onchange" and "onblur" as I can and will use actual functions to clean-up what-ever I can't do in line using "onsubmit". Again, thanks so much for your help!