I filed this bug but didn't get much traction.
Bug 36857 – Can not motify / single instant of reoccurring meeting from different timezone
I thought was kind of strange because I believe this is a critical bug that impact the basic calendar function.
The problem is not able to modify single instant of on-going meeting including deleting (canceling) one. The is on all AJAX clients include Zimbra Desktop. Is modify meeting from different timezone not a big requirement for most people?