IconVault's Countdown Timer counts down to any date and time you pick, or runs a duration timer you set in hours, minutes and seconds. The live display ticks every fraction of a second, and you can pause, resume and reset at any point. Quick presets for 10 minutes, 30 minutes, 1 hour, 2 hours and 24 hours start a timer in one click.

When the countdown hits zero you get an audible sound alert plus an optional browser notification, and you can copy shareable text describing the countdown. It runs fully in your browser, needs no account and is free. Note that background tabs may throttle timers, so keep the tab open for exact timing.