1. In Registry Editor, go to following key:
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\ImmersiveShell
2. In right-side pane, create a new DWORD UseWin32TrayClockExperience and set its value to 1
That's it. It'll immediately disable the new Clock and Calendar UI and restore classic UI in system tray.
If you want to restore new Clock UI, delete the new DWORD UseWin32TrayClockExperience.