By skyforbes I've seen a few posts looking for assistance in doing this, given that all the information online points you to Date & Time settings in the "Settings" menu, and recently Microsoft seems to have removed the ability to add clocks for additional time zones in that area.
But I found a fix for this, if anyone is looking.
From the Start menu, I typed in Control Panel to get the older Control Panel Interface.
In there, go to Clock and Region, then the option for "Add clocks for different time zones" is available under the Date and Time header. As it used to, it still only allows two additional clocks to be added, but for those of us who have friends/family in other timezones it remains the most helpful way to keep this information at hand.