Reliable Timing of Marathon Swims

As a marathon swim observer you have several responsibilities, but a big one is timing. Three critical details in your observer log are:

  • Swim start: date (year, month, day), time of day (hours, minutes, seconds), and time zone
  • Swim finish: date (year, month, day), time of day (hours, minutes, seconds), and time zone
  • Total elapsed time: hours, minutes, and seconds

This may seem trivial - and that’s probably why it gets botched up so frequently. Here’s the thing: You can mess up a log entry or a water temp reading or mis-spell someone’s name. Erase, cross it out, or start a new log sheet.

But if you mess up the timing of the start or finish - it’s gone! Never to be recovered.

Here are some suggestions to ensure you get an accurate and precise time of any marathon swim you are tasked with observing.

How to Time a Marathon Swim


Bring two timepieces which display the time of day. Any old watch will do, with the ironic exception of fancy automatics (e.g., Rolex, Omega) which can lose or gain significant time even in a single day. Designate one timepiece as the primary, one as the backup (in case the primary is lost or runs out of power). Note: Your smartphone is a timepiece!

Synchronize both timepieces with official NIST atomic time. Here’s a handy link: Note: your smartphone is likely automatically synched with atomic time.

I also recommend bringing a stopwatch (chronometer) - but this is not essential.

The Start

As the observer it is essential that you position yourself so you can actually see the swimmer starting. At night, or in rough shore conditions, this can be tricky. Coordinate with the pilot, crew, and swimmer to ensure you have visual contact with the swimmer as (s)he enters the water.

If you, the swimmer, crew, and pilot are able to coordinate so the swimmer enters the water on exactly the hour or half-hour, this will make the subsequent timing of feed stops and log entries much simpler. For half-hourly feeds, it’s easy for everyone to remember the feeds happen on “60” and the “30.”

Here’s Sarah Thomas beginning her 67 hour (16 minute, 12 second) swim in Lake Champlain at exactly 08:30:00 am.

Otherwise: when the swimmer’s feet enter the water, check the time of day on your atomic-synchronized timepiece, and immediately write it down in the observer log.

Also recommended: Have a second person start the stopwatch at the same time. Use verbal signals (“Start!” or “Go!”) to coordinate this. Why a second person? In my experience, trying to log the time of day and start the stopwatch at the same time is distracting and error-prone.

During the Swim

Watch the swimmer, monitor their well-being, record the water temp, stroke rate, and all those delicious details.

But here’s the important part about timing: because it’s based on the time of day - which can always be recovered - you don’t have to worry about the stopwatch. What if someone bumps it and stops the timer? What if it falls overboard? What if it runs out of battery power?

It doesn’t matter. The official elapsed time derives from the difference between the start time of day and the finish time of day — not the stopwatch.

The Finish

Again, as the observer it is essential to have visual contact with the swimmer exiting the water (or touching the cliff face, etc.). Again, this can be tricky at night or in rough shore conditions. Prepare for it, coordinate with the team, and make it work.

When the swimmer clears the water beyond the high tide line (or touches the cliff face), check the time of day on your atomic-synchronized timepiece, and immediately write it down in the observer log.

If you wish, stop the stopwatch, and confirm your elapsed time arithmetic. But as you know by now, it doesn’t matter. You may be tired and mess up the date/time subtraction. That’s OK, you can fix it later. Or the sanction org will notice your error a few days later, and fix it in the ratified results. As long as you have the date and time of the start and finish, it’s all good.

Simple, right?!