Your screen grab shows the Settings → System → General page.
Just to be sure: do you also know the User Profile settings?
See: Your account profile
Click on your user name at the very bottom of the sidebar.
There you can set the Time format, Date format etc for each logged in user.
I looked the settings under Settings–>People–>People and / or Settings–>People–>User
That was probably not the right place
But still I ask myself / wonder why there are different settings for the user?
One time ín seidebar bottom left and another time under the above mentioned settings.