appointment time is automatically moved to another time

Hello, I have this issue with a client:

A user creates an event at 14:00 and invites other 2 users the event.

These users confirm the appointment, but to confirm the appointment time is automatically moved to another time

* It happened several times and for different users.

What steps must I take to see what happens?

thanks

zimbra: 8.6
zextras 2.0.1