Some seriously harsh critics here! Personally, I think it's amazing that he could get them even within 5 minutes per day. For woodwork, that's some pretty impressive precision. Let's do the math:
5 minutes per day = 5 minutes per 24 hours. Rounding, that's 1 minute every 5 hours. Sixty seconds for every 5 X 60 X 60 seconds. Simple math makes that 1 second for every 5 X 60 seconds, or 1 second for every 300 seconds. Certainly not the standard we are used to, but given the method of production I'm impressed. Say you wore one to work and set it when you arrived at the office. Assuming a nine hour day including lunch, by the time the work day was over the watch would be roughly two minutes off. I could live with that.
Another way to look at it is 12 seconds per hour. 12 X 9 = 108 seconds in a work day.