5.1.1.2.1 Addition and 5.1.1.2.2 Subtraction: Date add/sub Duration is DateTimeOffset - but in which time zone?

Rules are actually present in XML Schema 1.1 Part 2: E.3.3 Adding Durations to DateTime. Rules: 1) Convert date to datetime (in any timezone) with zero hh:mm:ss 2) Add/subtract duration 3) Convert to date by dropping fractional days and timezone Thus today + 1 second is today. Today minus 1 second is yesterday.