A day-date complication is a feature that displays both the day of the week and the date, usually through one or two small windows on the dial, in addition to the regular time. It's one of the most common complications on both quartz and automatic watches because it adds real, everyday usefulness without adding a lot of mechanical complexity or cost.
How It's Actually Set and Adjusted
Most day-date watches let you set both functions using the crown, either by pulling it to an intermediate position or by using a quickset function that jumps the date forward without moving the time hands. Watches without a quickset feature require you to move the hands all the way around to advance the date, which is slower but not complicated. Either way, you'll need to manually adjust the date after any month with fewer than 31 days, since the mechanism doesn't know that April has 30 days or that February is even shorter, and it will simply keep counting to 31 unless you correct it yourself.
Why Some Watches Skip It Entirely
Plenty of well-regarded watches leave off the date window entirely, often as a design choice to keep the dial clean and symmetrical, since a date window breaks up the layout and can throw off the balance collectors and designers care about. It's a tradeoff between usefulness and simplicity, and neither choice is wrong. If you like glancing at your wrist and knowing the date without checking your phone, a day-date function earns its place. If you'd rather have an uncluttered dial, it's easy to find great watches without one.