The &ZTIME variable is on the usually on the LOGO screen.
TPX gets the time using a standard STCK instruction (store clock). This is using an IBM routine not TPX to get the time.
The store clock instruction is storing the actual time, GMT/UTC as set by HMC (Hardware Management Console). The local time is derived from a CVT (Communication Vector Table) value which contains the value converted from PARMLIB as to the (plus or minus) difference in hours from GMT/UTC for local time.
Check if you see the same time that is displayed in TPX being displayed anywhere else.
Does the VTAM MSG10 screen (logon) show the same time as TPX?
Check what has changed in your system since the time differences have been noticed.