Keep your old passports
You need to list all the trips you have taken outside the US since you became a permanent resident. Saving your passport will let you track all those dates. Immigration looks at the dates to make sure you meet the residence requirements for citizenship and that you did not interrupt your residence for citizenship purposes or give it up. Saving your old passports is far better than guessing the dates of your trips. If your country of citizenship keeps your old passport when you get a new one, take a copy the pages of your old passport before you replace it.