Cfars app for receiver

I am interested in building an ak and need to apply for a unique serial number

On the cfars app it is asking for a date of manufacture.

If I need the unique serial number prior to manufacturing and it takes up to 90 days to receive the serial number and then I have 10 days to engrave.

How do I know a future date of manufacture.

In essence what date do I enter on my application. I’m probably overthinking this