By adding this DWORD value, you can add the display of seconds to the Windows 10 taskbar clock. You’ll have to restart Explorer in order for the changes to take effect.
To remove the seconds, simply remove the DWORD from the registry, and restart Explorer again.
If you are unsure of how to restart Windows Explorer, you can check out this post for instructions.
Windows Registry Editor Version 5.00 [HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced] "ShowSecondsInSystemClock"=dword:00000001
I’ve included .reg file (attached to this post, above), for your convenience. Save it to a known place on your hard drive, then double-click to enter it into the registry.