This is very similar to the closed
MDL-2366 submitted by another user (which disappeared on its own without definitive resolution apparently).
I have user and site timezone settings all set to GMT+12.
The server is in GMT-6.
The calendar entries' date line displays the selected day correctly but follows it with itself and precedes it with two days previously. In other words something like:
Sunday – Tuesday – Tuesday
Server's running on Linux, users on WinXP.