Your time as a permanent resident begins on the date you were granted permanent resident status. This date is stated in your Permanent Resident Card (green card). It is indicated on the bottom of green card near words “Resident Since:”.
However, don’t be surprised if your “Resident Since” date is prior to the date that you received your actual green card. For example, if you were granted an immigrant visa at the U.S. embassy in your home country and the green card was mailed to you at your U.S. address, you’ll notice that the date will be the actual grant date.
And if you do not have your green card in your possession, you may be able to determine your time as a permanent resident from another document. Search for the I-797 Notice of Action or consular letter that granted your permanent resident status. The date of the letter is the date you became a permanent resident.