So, what is a reasonable expectation of accuracy from a wristwatch?

 As a guide a modern mechanical watch could vary between +/- 15 seconds a day at worst and +/- 2 or 3 seconds a day at best and both figures are generally within tolerance depending on the type of watch and age. For example an automatic watch is usually more accurate than a hand-wound mechanical but quartz watches are better than any mechanical watch by a large margin. As a rule a quartz watch could be as accurate as +/- 0.01 although +/- 2 seconds a days is within the acceptable limits of most manufacturers.