Have you done any migration testing since the time change? We're seeing a lot of appointments coming across with the appointment shifted an hour early. We suspect the way GW is handling the time change transition may be the cause but we're still investigating.
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We're also having problems with duplicate appointments. Here are a couple of bugs that may describe the problems we're seeing.
Bug 25711 - Groupwise:Appointment duplication on migration. Bug 25709 - Groupwise:Mismatch in Appointment time after migration.