Never answered your post, sorry. The Priority Date in case of an I-130 is the receipt date of the application (you'll see that on the approval notice). So if you applied in December 97 and it got approved in April 2002, your priority date is December 97. And once the running priority date (i.e. visa bulletin) hits December 97, you can file the I-485 (or whichever application is next for you).