If you see "The Month parameter must be in the range 1 through 12." thrown using the 1.0 version of the Exchange Managed API when dealing with non-DST timezones, then you should look at upgrading to the 1.1 version of the Exchange Managed API - the issue should be fixed in that version.

In OWA 2003, there were OWA settings mirrored in properties on the root folder of mailboxes which WebDAV could get to. There was limited support for accessing a few of these properties with WebDAV. The online docs still state that these properties are there, however this is not really true for OWA 2007...

CDO 1.21 Timezone information is stored in a a property inside of the user's mailbox and is CDO 1.21 specific. It is set any time a CDO 1.21 session sets the time zone and persists for that user until changed. This means it will be used by any subsequent CDO 1.21 session. OWA 5.5 will affect the timezone...