I have a clock on my taskbar which tells me the time here, and EST is just a mouse-scroll away at the bottom of this page
If I double-click the clock on my taskbar, and scroll down the time-zones, when the main window comes up, I can tell the time in Krasnojarsk, Ekaterinburg, Port Moseby, and Nuku'alofa.

When travelling, I just get it into my head the difference in time-zones before I go, and remember it.

That doesn't cause me any problems, but most digital watches, or travel alarm clocks, nowadays have multiple time-zones programmed into them.

Sometimes less is more, though