Please answer a question about the following article about Premier League:

The Premier League is broadcast in the United States through NBC Sports. Premier League viewership has increased rapidly, with NBC and NBCSN averaging a record 479,000 viewers in the 2014–15 season, up 118% from 2012–13 when coverage still aired on Fox Soccer and ESPN/ESPN2 (220,000 viewers), and NBC Sports has been widely praised for its coverage. NBC Sports reached a six-year extension with the Premier League in 2015 to broadcast the league through the 2021–22 season in a deal valued at $1 billion (£640 million).

When will this extension end?
through the 2021–22 season