HTC Clock is an app that might've been bundled with your device if it was made by HTC, but anyone can download it. It's a suitable app for keeping track of the time, of how long you've been running, or how long something takes. Of course, it can set alarms too.
Previously, it used to come in black to best match the rest of the Android interface. The latest version has a rather bright green-and-white colour scheme instead.