Returns the hours component of an xs:dateTime.
fn:hours-from-dateTime( $arg as xs:dateTime?xs:integer?If $arg is the empty sequence, the function returns the empty sequence.
Otherwise, the function returns an xs:integer between 0 and 23, both
inclusive, representing the hours component in the local value of $arg.
The expression fn:hours-from-dateTime(xs:dateTime("1999-05-31T08:20:00-05:00")) returns 8.
The expression fn:hours-from-dateTime(xs:dateTime("1999-12-31T21:20:00-05:00")) returns 21.
The expression fn:hours-from-dateTime(fn:adjust-dateTime-to-timezone(xs:dateTime("1999-12-31T21:20:00-05:00"),
xs:dayTimeDuration("PT0S"))) returns 2.
The expression fn:hours-from-dateTime(xs:dateTime("1999-12-31T12:00:00")) returns 12.
The expression fn:hours-from-dateTime(xs:dateTime("1999-12-31T24:00:00")) returns 0.