As the trend is towards multiple devices, more and more mobile clients and apps gain access to our calendar to make our life easier and more comfortable. Also the way we work changed and multiple assistants maintain a single mailbox. In general everything works across multiple platforms and clients. But what when suddenly a few people turn up at your office as all of them have the appointment....except you? In this session you will learn how you can debug Exchange Active Sync devices and use existing built-in tools to examine the history of calendar items (e.g.: who and what was changed by which client) either on-premises (e.g.: DatabaseEvent, IIS logs) or in Exchange online (e.g.: EWS, Get-CalendarDiagnosticObjects). Did you know that EAS is the only protocol, which logs all the actions in IIS logs? There is not always the need for a Microsoft support case. Besides the Cmdlets and built-in there are also community-build tools.