Sitting with your face buried in a computer screen can really make you lose track of time, especially if you're working in a window-less office. But you can create your own "window" with your computer wallpaper and keep track of the passing of time in a clever way—no numerals required.
Just using your computer's built-in rotating wallpaper setting, you can use your background to keep track of the passing hours, just like a bell tower. Each time the scene changes, you'll know another 15 minutes, 30 minutes or an hour has passed, depending on how you set it.
If you want to get really creative (and mimic the "window" we're talking about in the intro), pick out 24 stellar wallpapers to represent each hour of the day. A quick Google search will return images of gorgeous day- and night-time scenes from around the world.
If you're a budding photographer, you could even shoot the same scene at different hours of the day for a time-lapse-style wallpaper. It's the perfect background for your work to keep you on top of your time management.