ClockMaster - Time Regulator 4+

This app helped me resetting a grandfather clock I had to disassemble and clean & oil. The iPhone ear buds work far better than the internal Mic which is almost useless. A clip-on piezoelectric pickup would be much better still.

They only thing I don't like is that only an absolute delta is shown. It works be most helpful if there was some indication of which half of the cycle was longer/shorter when adjusting the balance. I ended up using a flashing metronome to tell me whether I had to tighten or loosen the pendulum leader offset. Lose one star for this deficiency.

I've tried a couple other apps but this is spot on. I would highly recommend.

I used this to calibrate my cuckoo clock. It picked up the pendulum clicks just fine using the internal mic, with no need for an external mic or sensor. I'm definitely no horologist, but this seemed to do the job. When I had a problem due to an iOS version issue, the developer responded within a few minutes and had a fix quickly.

It would be nice to have plots to look at drift rate and periodic error. These are available in the Pro version, but even in that it will not make plots - just allow you to export the data to a file to be viewed elsewhere. Would be better to have the plots right in the app.

This has become my go-to app when I'm fine tuning a clock, whether it be a perfect tick-tock or getting it timed to as little gain/loss per day. It has been frustrating in the past as the app didn't support my iPhone 5-5S, I had to resort to an old 4 for it to work. Just tried it on my 6S+ and had no problems!
I haven't upgraded to pro yet but I am interested as I have a piezo microphone with a 3.5" jack and instead of using my small amp next to my phone I'd love to be able to plug it into my phone and reduce the background noise. $20 is hard to swallow though. Now if it were say $5...