How to Show Seconds in Windows 10 Taskbar Clock
To display seconds in the Windows 10 Taskbar clock, follow these steps:

Start the Registry Editor (regedit.exe)
Go to the following registry branch:
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced
 Save
In the right-pane, create a DWORD (32-bit) value named ShowSecondsInSystemClock

Double-click ShowSecondsInSystemClock and set its data to 1
Exit the Registry Editor.
Log off and login back in. Or restart the Explorer shell for the change to take effect.

This registry edit works in Windows 10 v1607 and higher — tested up to v20H2.