This means the announcement will be January 9, 2018, 220-days before his 55th birthday.
In light of this news comes January 5, or 1/5, let us not forget that the Raiders played in Super Bowl 15. You can also write the date as 5/1. Super Bowl 55, will be the 51st of the modern era.
There's also a connection with 'Oakland Raiders', 'Gruden' and '55'.
As for the 9/1 announcement... consider 91 is the 13 triangular number...