How does Zimbra manage updated Appointment?

[gambo]

I have a problem. It seems that Zimbra ignore DTSTAMP value.
When i try to retrieve appointments by timestamp range with caldav4j i get the whole list of appointment without timestamp filter.
So i begin to think that Zimbra doesn't use Timestamp to understand which appointments have been updated. Anyone can explain me how zimbra works?
How can it understand which appointments have been modified in order to synchronize with devices or clients? Does it use something like a flag?
Can i retrieve appointments with flag=true with SOAP interface?
Thanks

[gambo]

With SOAP interface and GetApptSummaries i can retrieve appointments in a specified time range. But if i want to retrieve appts by timestamp? Is there a way to do this?