When you say "events", do you mean full-day items (as opposed to an item scheduled from, for example, 9am to 11am)? If so, perhaps these KBs apply:
KB20341 Calendar entries turn into all-day events, and the end date appears one day earlier on the BlackBerry smartphone after synchronization
KB26533 All day appointments created on a BlackBerry smartphone in a GMT+ time zone synchronize to the GroupWise client one day earlier
Good luck and let us know!
Occam's Razor nearly always applies when troubleshooting technology issues!
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