Archbishops in the Church of England, as a rule, must retire on or before their seventieth birthdays. However, in special circumstances, the Queen can grant an extension of up to one year.
Archbishop Sentamu will be 70 on June 10th 2019. In the ordinary course of events therefore he was scheduled to retire on June 10th, 2019, unless he announced he wished to go earlier.
However, the Queen has granted an extension of almost one year, allowing the Archbishop to defer his retirement until the Sunday before his 71st birthday, Trinity Sunday, 7th June, 2020.
The "news" here is not so much that he is retiring in 2020, as that he is not, as originally expected, retiring in 2019. The real news is the extension, but obviously the date that someone is retiring is considered more interesting, and newsworthy, than the date that he is not.
As a rule 20 months notice (October 2018 to June 2020) would not be usual.