Users on Windows 10 v1607, can open Windows Registry, navigate to the following registry key

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced

create a new REG_DWORD, name it ShowSecondsInSystemClock and give it a value of 1.

source: https://www.thewindowsclub.com/display-seconds-in-taskbar-clock-windows