The "combined date and time representation" is broken in Switzerland and all other locales which switch to day-light-saving time and back, on those days which are 23 hours or 25 hours long instead of the usual 24 hours.
Acknowledged

This problem is most easily avoided by always using UTC for the "combined date and time rep-resentation".
Clarify the behaviour on transitions to daylight saving time and back.

Part 4,
3.17.4.3

te

Proposed Disposition of DIS 29500 Comment CH-0010 (Modified: 2008-01-02) While we understand the comment, and agree that there is no way in SpreadsheetML to designate or differentiate between “standard time” and “daylight savings time” in those locations where time shifts are used, we do not plan to make any changes to the specification to support “daylight savings time.” SpreadsheetML assumes that all times are specified using UTC or using time zone offsets from UTC as specified by ISO 8601. If ISO 8601 is updated with a standard representation for differentiating “standard” and “daylight savings” time, we recommend that the SpreadsheetML specification be updated to match. Note that we do understand the importance of supporting ISO 8601 date and time formats (including time zones), and have added support for specification of UTC dates and times using an ISO 8601 format in response to several other comments, including AU-0017 and JP-0071. Similar Comments: GR-0024 , IR-0044 , IR-0045