There are phone apps you can use to do that automatically. I think most of them are designed for mechanical watches, but they could probably handle a clock tic

There are phone apps you can use to do that automatically. I think most of them are designed for mechanical watches, but they could probably handle a clock ticking, too, unless the sound profile is weird. At 240 fps, iPhone slo-mo would need about a minute or three of filming to get a decent measurement of accuracy, so audio is generally more convenient.