It is assumed that the units are the same. So 0.00002 seconds per second. Or 0.00002 months per month.
There are 2,592,000 seconds in 30 days. So accuracy is about 50 seconds a month. Call it a minute. Pretty poor for a quartz watch.
Thanks.
My mechanical watches have never needed servicing, except the time when I broke a crystal watchface.
Still working does not mean still working well.
Like most mechanical things they need maintenance to work at their best. They may keep working without maintenance but they will not work as well over time (a car being a common example).
Why you do not need a chronometer watch:
Though a watch is much easier to read when you’re outdoors on a sunny day.
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