yinyinzhi is right.

You need to check the USCIS website every month to see whether the I-485 is filed based on the Final Action Date or the Filing Date. Normally, the Filing Date is used for the people who apply for the immigrant visa outside US. Because immigrant visa application process takes more time, USCIS set the Filing Date for the applicants to submit the documents to NVC earlier for processing. Of course, sometimes, the Filing Date may also be used for I-485 filing.  Check this website for the I-485 filing date each month:
https://www.uscis.gov/visabulletininfo