Windows 10 does have a clock baked in that can be used on the desktop, it's not the most intuitive thing to find but it is there.
1] type alarm in the search bar, alarms and clock app should pop up
2]open this and there should be 4 tabs, one of them for the world clock. Add a clock there
3] the time zone should be set to your system time zone, if not go to settings- time and language- data and time to set the local time or right click the time and date at the bottom of the screen in the app to set the time
4] all you need to do to keep the clock on the desktop is drag the app from from the top right corner of the screen to resize it to the middle of the screen then you can move it wherever you want it
If you find you don't like the look of that clock there are plenty of 3rd party desktop clocks and widgets out there. If you want to keep it in the MIcrosoft realm just look in the MIcrosoft store and search clock