According to this TechNet question and answer, when trying to save a project that was originally created locally to Project Server, and you get the error "Enterprise standard calendars are needed", you can avoid getting that error by enabling "Allow projects to use local base calendars" for the PWA instance.
Now, when you save the project and project server is set not to allow local base calendars (PWA/Server Settings/Operational Policies/Additional Server Settings/Allow project to us local base calendars), then this popup you mentioned will appear and as result, the local project calendar will be replace with the project server standard calendar. If those are different, then dates get changed (more or less holidays). The easy workaround is to allow the use of these local base calendars (PWA/Server Settings/Operational Policies/Additional Server Settings/Allow project to us local base calendars, restart client).
I had my Sys Admin follow the instructions here to enable that setting for the PWA instance I am working with.
I restarted my Project client, connected to my PWA, and tried to save a local project to the server, but I am still seeing the "Enterprise standard calendars needed" error.
How long before that goes away? Do we have to do some sort of reset on the server side?