Understanding AC21 180 day rule

[murthy_questions]

AOS - Pending with receipt date of 10/26/20, did not received appnt for biometrics yet i.e. no EAD

I140 (EB2 to EB3 downgrade in Oct. 2020) - Approved

I recently received a offer in Similar Occupational Classification so trying to understand if I MUST wait until Monday 04/26/20 to join new employer (understand that H1B transfer approval notice take until March/end or so even with premium processing; not sure if employer would be willing to wait any longer after that)

As per USCIS, to file I-485J, AOS should be pending for 180 days or more since the AOS receipt date.  Does it have to do anything with my joining date with the new employer? i.e.

Should 180 days be counted as Joining date@new employer minus receipt date OR
Should 180 days be counted as Filing date of I-485J@New Employer minus receipt date

My current employer confirmed that they do not revoke AOS or I140 application after employee leaves

Edited February 1, 2021 by murthy_questions
For more clarity