Date Validation And Calculation Of Months

I have this form where in i calculate the number of months
between 2 dates.It calculates only if both the textboxes have values.
And then i display this in another text box.
The user has to enter the date in DD/MM/YYYY format. user can select
date from calender popup. Apart from selecting the user can also type in
the date.
I am using a function which splits the date entered by the user
into DD MM YYYY and then compares each of these values to see
if its in the valid range like day is between 1-31, month between 1-12
etc.
But i m having a problem. If the user enters characters instead of digits,
my system accepts it in the first box without giving error and then when
the user selects a proper date in 2 textbox its gives an error: 1 is null
or not an object. this error comes at the place where mt function splits
the entire date string into DD MM YYYY.
Could someone please give ne a solution, a way to format the date so that
i let the user enter first 2 digits and then a "/" and then 2 digits and then a
"/" and then 4 digits only.