Or you can tell the mindless bureaucrats to get their lazy azzes and get under your cars and look at the build sheets glued to the gas tanks.

Most unhelpful suggestion just hoping it makes you laugh a bit. Your 90 ZR..betcha a October, 1989 build date. Oh...when you get the build sheet from NCM looked at top middle of the sheet and you will see
DATE: 89/xxx and in my case my 90 #566 is DATE: 89/297 which means 1989 297th calendar day... so what ever you number is just count from January 1, 1989 and that is your build date.
This is what a build sheet looks like:

Michael