I am in the midst of creating an invoice using MS Excel 2011 (Mac Osx). I have populated one cell with the name 'Invoice Number' (H6) and would like I6 to be the cell which auto generates the invoice number. I have to comply with office standards so the formatting would be yyyymmdd-VT0001. I would like the ability for this number to increment itself by 1 every time a new invoice is created. And on that note, if you have an auto generated invoice number, please advise of the following;
a) once it generates the invoice number and let's say a week has passed and you realized you've made a mistake, will it try and create a new invoice number or will the invoice number remain unchanged?
b) is it better to create the formula and then save as a template? I'm just thinking for every time an invoice is created then I'd have to locate the last one which is a little long-winded.
c) at what point does the auto generated invoice know to increment itself?

Sorry, I'm a slight novice and the sheer volume of info on the net is misleading to say the least.

Thanks in advance.