What version of Exchange?
Basically timezones are an horrendous issue in any calendaringapplication - in fact date and time calculations are a disgustingly
complicated thing in general. If you search for "evolution-ews
timezone" on Google you get some hits on blog entries and bugzilla
reports on problems and some hints on how to deal with them. Basically
you need to make sure Evolution and Exchange are both using the same
timezone.