Visa validity/duration

Hi, I have an F visa for China which has an issue date of 04/07/2023 and a validity expiry date of 04/10/2023. The duration is 90 days stay allowed in China. I entered China on 17.07.2023 so according to my calculation, if I am allowed 90 days in China, I can stay until 15th October. However someone here is saying I have to leave on 04/10/2023 because that's when my visa expires but I believe that is the expiration of the validity of the useage of the visa i.e. I had to enter China before that date to be able to use my visa but once I enter China within the validity period of the visa, no matter how far into the validity, I can still stay 90 days regardless of the validity expiration date. Please can you clarify this. Thank you