Is there any difference in accuracy on a 400 vs 1000 day clock? My first thought is that since the mainspring of a 1000 day clock will be tighter and have more consistent power draw, over the course of a year, you could probably dial it in to be more accurate than a 400 day clock. Or at least as accurate as a 400 day clock is over 3-4 months. My second thought is that none of this matters, and getting a clock that is within several seconds a day is generally considered high accuracy on a mechanical clock, but that still places you 5-10 minutes off every 3 or 4 months, and 400 or 1000 day, doesn't matter, that is a very hard goal to achieve regardless of how consistent the mainspring is. And yes, yes, I know "if you want accuracy, go quartz", but I think there is something very satisfying about getting a mechanical movement as dialed in as possible. To me it's part of the allure of the mechanical - that this is just classic newtonian motion at work yet it can still achieve surprising accuracy.