Is processing time starts from the day the day they received my form or the day my green card expire,

I-751 applications are taking 7 to 9 months. Your status is automatically extended one year when the I-751 is receipted (travel and work authorization). Did you submit all of your supporting documents with your application?

The processing time starts from the day they received and accepted your I-751.

Processing time starts from whenever the USCIS receives your I-751.

When they received your application is the relevant start time for the clock.

The processing time starts at the time the USCIS receives your I-751 application. You can file the same 90 days prior to the expiration of your conditional green card. Make sure that you don't file it earlier. Best of luck and God bless.