According to the site in the related link, it is 12 hours, 14 minutes and 53 seconds.
21 September 2003 was a Sunday.
21 September 1943 was a Tuesday.
21 September 1970 was a Monday.
21 September 1982 was a Tuesday.
September 21, 1968 was a Saturday.
September 21 1965 was on a Tuesday.
21 September 1971 was a Tuesday.
September 21 1969 was on a Sunday.
21 September 1977 was a Wednesday.
September 21, 1980, was a Sunday.
September 21 2010 was a Tuesday.
September 21, 1962 fell on a Friday.