An average digital watch is very accurate. Usually you won’t find more than a variance of 15-30 seconds in a month, and it’s rare to even have that much difference.
The difficulty in determining the accuracy of a digital watch though comes in the potential for affecting the quartz crystals inside.
The way a digital watch tells the time is by measuring the vibrations of a quartz crystal to a very precise degree. As a result, if the vibrations are affected by external factors, then the time measured by the digital watch could be slightly inaccurate.
The biggest indicator of whether or not the accuracy of the watch will be affected is the quality.
If you invest in a poor quality digital watch, with sub-par craftsmanship, then you will have to live with slightly inaccurate time readings. Though, this isn’t all too common.
Another factor which can affect the accuracy is the temperature of the environment you’re in. If the temperature is high, the quartz crystals tend to vibrate a little faster, and the opposite is true if the temperature is very low.
Air pressure can have the same effect on the quartz crystals, with increased air pressure leading to slower vibration and vice versa.