How to get timesheet information in a javascript (Project Server 2013)

Hi all,

In Project Server 2013, I'm trying to customize the Timesheet ribbon. This far, I've been able to add a new button and tie an event to it. All is working well. This is all done using elements.xml and a javascripts.

Now the problem is that I want to get some informations about the timesheet (the Guid for example) and, for now, I've got no success. I know that there is an object called PS.TimeSheet, but when I try to use some PS.TimeSheet's methods I get an error.

Note that i'm in the page http://servername/pwa/timesheet.aspx (there is not Guid in the address) and all the treatments should be done in the javascript (no call to another aspx page)

Thanks !