Go to online-stopwatch.com, and select Count Down.
From the numerical interface, select the time you want. For example, if you want to count down 59 minutes, you must select five then nine then zero and then zero. When you're done, click Set.
Click Start to view the time counting down to zero. Use the Pause button, if necessary, to stop the time. Leave your sound on if you want to hear the alarm sound when the time is up.