A string representation of this duration using ISO-8601 seconds
based representation, such as PT8H6M12.345S.
The format of the returned string will be PTnHnMnS, where n is
the relevant hours, minutes or seconds part of the duration.
Any fractional seconds are placed after a decimal point i the seconds section.
If a section has a zero value, it is omitted.
The hours, minutes and seconds will all have the same sign.
Examples:
"20.345 seconds" -- "PT20.345S
"15 minutes" (15 * 60 seconds) -- "PT15M"
"10 hours" (10 * 3600 seconds) -- "PT10H"
"2 days" (2 * 86400 seconds) -- "PT48H"
Note that multiples of 24 hours are not output as days to avoid confusion
with Period.
Diagram: Period