It sounds like the cell containing the date has the formula
=TODAY() in it, this is fine for producing the initial invoice but not
if someone asks for a copy later as I suspect is your problem.
If you open the template and go to the date cell delete the formula and
save the template, when you open up the template later you should be in
the cell where the date needs to be entered manually.
The other method is to copy the contents and paste special, values
before you save each new invoice.
Either method will then have the correct invoice date in, but
unfortunately both methods do require manual input. You could record a
macro which prompts you to enter the date which would at least cut the
probability of not putting a date on the invoice.
Hope this helps.
Mike
