Best time to renew passport before trip in September?

Renew now. That is the best time. If you don’t mind the fee, pay for expediting to make the waiting for its return less tortuous.

I always feel better knowing that things are set for awhile and don’t like leaving things to last minute. You know you don’t have any immediate plans, so that makes now a good option.

Going by the rules you don't have to renew it since your departure frrom the Schengen Zone is 90 days before it expires
https://travel.state.gov/content/travel/en/international-travel/before-you-go/travelers-with-special-considerations/schengen.html#:~:text=A%20U.S.%20passport%20(with%20applicable,departure%20from%20the%20Schengen%20area.

If your departure from Europe was in January 2024 then you should renew it. For example my last trip was in May 2022 and my passport was set to expire October 2022, I renewed it June 2022. But if this is worrying you it can't hurt if you have the money and time to do it now.

You can renew at any time, no penalty. I suggest renewing it in plenty of time to have your new passport when you travel. Keep in mind you’ll get a new passport number, so you may need to update it for plane tickets, etc.
https://www.usa.gov/passport#item-34907

No penalty for renewing a little early, you can get that task out of the way and you won't have to worry about cutting it close.

Unless you anticipate a need for your passport in the next few months, I'd go ahead and renew it now. September is not that far away.

Thanks. I do not mind the fees as it will lessen my worry. Since trip will be in September, I think I have enough time not to have it expedited. Thanks for the reminder, I have global entry and the priority pass. Does it mean I will have to make another appointment to update my global entry card as the passport # and expiration date will be different after I got the renewal?Thanks.

You should be able to update your passport information in your online Trusted Traveler account. You don't have to upgrade the Global Entry card. What's important is that the new passport information is in your Trusted Traveler profile. That's the what the GE machines access.

If you aren't able to upgrade your passport number online, just stop by any Customs and Border Protection office and they can do it for you. No appointment necessary.

Thanks a lot. Good to know. I will renew it by this February.