There was a time where you could not display the seconds within the system clock, and even today it is not set by default within Windows 10.
This article from 2003 explains the reasons why there was no seconds displayed in the system clock: https://blogs.msdn.microsoft.com/oldnewthing/20031010-00/?p=42203
Starting with Windows 10 build 14393 you can now place the seconds in to the system try clockk.
Head in to the registry (regedit)
New > DWORD (32-bit) Value.
Set the Value Name to: ShowSecondsInSystemClock
Value set to 1
Then reboot the computer or log out and back in again.
You should now see the seconds ticking away in the system try clock.