DateTime.Now vs system time
You can check the following msdn link. It clearly states the following :
Notes to Callers: Local time zone data is cached after CurrentTimeZone is first used to retrieve time zone information. If the system's local time zone subsequently changes, the CurrentTimeZone property does not reflect this change. If you need to handle time zone changes while your application is running, use the TimeZoneInfo class and call its TimeZoneInfo.ClearCachedData method.
is this documented anywhere?Yes, as stated above.
why isn't the timezone change picked up while the application is running?Because the timezone information is cached.
can I do anything so that the app picks up timezone changes while it's running?Yes, by using the TimeZoneInfo class and calling its TimeZoneInfo.ClearCachedData method.