I just found this method that works quite well.
- Right click on any taskbar, then uncheck
Lock all taskbars.
- Move the main taskbar (the one with the system tray on it) to a screen of your choosing. You can do this by dragging the whole taskbar.
- (optional) Right click on any taskbar and check
Lock all taskbars again.
While playing games, the clock will remain visible at all times.
Tested and works on Windows 7, 8/8.1, and 10.
Starting with Build 14328, the clock will remain on all taskbars in Windows 10.
Taskbar clock on all monitors: We heard a lot of requests for this from our gaming community. We’re happy to announce that for the first time since the taskbar was created, if you have multiple monitors and your taskbar set to show on all displays, the clock will now be visible on each Taskbar.
Edit 2016-08-05: This feature has been shipped with the Windows 10 Anniversary Update (version 1607)