Here are the official examples:

"PT20.345S" -- parses as "20.345 seconds"

"PT15M" -- parses as "15 minutes" (where a minute is 60 seconds)

"PT10H" -- parses as "10 hours" (where an hour is 3600 seconds)

"P2D" -- parses as "2 days" (where a day is 24 hours or 86400 seconds)

"P2DT3H4M" -- parses as "2 days, 3 hours and 4 minutes"

"PT-6H3M" -- parses as "-6 hours and +3 minutes"

"-PT6H3M" -- parses as "-6 hours and -3 minutes"

"-PT-6H+3M" -- parses as "+6 hours and -3 minutes"