A watch's accuracy is its rate: the seconds it gains or loses every 24 hours, written in seconds per day (s/d). The most reliable way to find it at home is to compare the watch with an exact reference twice, about a day apart. It works for mechanical and quartz watches alike.
Step by step
- Use an exact reference. Horology's Time Check keeps atomic time from Apple's time servers.
- Watch the seconds hand and tap the moment it reaches 12, then confirm the hour and minute on the dial. A tap a fraction of a second late is fine: it evens out over the days.
- Wear the watch, or let it rest, as you normally would, and note which.
- About a day later, take the second reading the same way. Horology reminds you when it's time.
Working out the rate
The offset is how far ahead (+) or behind (−) atomic time the watch is at a reading. The rate is how the offset changes each day: the difference between two offsets, divided by the days between them.
| Offset | |
|---|---|
| First reading | +1.0 s |
| Second reading, a day later | +4.2 s |
| Rate | +3.2 s/d |
What's normal?
| Watch | Rate |
|---|---|
| Certified chronometer (COSC) | −4 to +6 s/d |
| Quartz | Usually within half a second a day |
For an instant reading of a mechanical movement, with its amplitude and beat error, use a timegrapher instead: see rate, amplitude and beat error, explained.