This simple Registry tweak will let you re-enable the old classic Windows 8.1 / 7 type Clock and Calendar in Windows 10.
Open your Registry Editor, press Win + R, type regedit and hit Enter. (Before editing the Registry, don’t forget to create a backup of your Registry files.)
After opening Registry Editor, navigate to the following key:
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\ImmersiveShell
Click on the Immersiveshell folder on your left hand side and create a new DWORD (32-bit) Value in your right hand side.
To create a new DWORD Value, right-click on the empty space of your right hand side, select new and click on DWORD (32-bit) Value.
Name it UseWin32TrayClockExperience. By default, the value would be 0. You have to set the value to 1. To change the value, double-click on UseWin32TrayClockExperience and enter 1 before saving your change.
Right after setting up the value to 1, your new Windows 10 Clock and Calendar will be changed to Windows 7 style Clock and Calendar.