How the date change mechanism work
As you can see on your watch, the hands move every single second at least (even the hour hand, even if you can’t see it with your naked eye). Conversely, the date stays still for at least 18 hours, and they need to change “overnight”.
The power is delivered by a tiny motor in quartz watches, and by the mainspring and gear train in a mechanical watch. Yet, the movement can’t just change the date instantaneously.
In order for the watch movement to be able to change the date “overnight”, it needs more power than the one it use to make the hands move. This is true for mechanical watches (automatic and manually wound) and quartz watches as well.
The watch movement has to somehow accumulate power to make the date wheel move forward. To do this, a special mechanism is engaged during the hours preceding and following midnight.
Depending on the type of movement your watch has, the date change mechanism will be different:
Instantaneous date change: the power is slowly gathered in a cam. When your watch approaches midnight (within seconds of it, actually), the power in the cam is released suddenly. This, in turn, will push the date indicator forward one day so quickly that you can’t see it moving with your naked eye. You can find this in pretty much every high-end watches.
Semi-instantaneous date change: the power is gathered in a spring that is mounted on a wheel. This power is then released when the power in this spring becomes bigger than the power of the spring-loaded date click.
Standard date change: a small prong or claw on a wheel grabs that date wheel and push it forward to the next date. This happens over the course of a few hours. You can find this mechanism in every affordable watch.
By the way, pretty much the same mechanism apply for the day wheel, if your watch happens to have one. Of course, this is not an in-depth explanation of how the date change mechanism work.
Also, not every watch has the same movement, and the way the movement accumulates power and makes the date wheel move forward might be a little different. The goal here is for you to understand the general mechanics of this system.
Why changing the date between 9:00 PM and 3:00 AM is bad for your watch movement
The reason why you could damage the movement by changing the date between 9:00 PM and 3:00 AM is simple.
Remember the date change mechanism a bit earlier? They all engage to accumulate power and torque beginning around 9:00 PM. And some watches with a standard date and day change mechanism are still engaged up until around 3:00 AM.
So if you use the quick date set system during those hours, you will apply a force to those gears that the watch is not supposed to handle. As a matter of fact, another system will drive the date wheel forward when you set it manually.
Doing so, you could damage the mechanism that automatically changes the date and day of the watch around midnight.
Don’t do it.
You could set the date at the wrong time maybe once or twice without harming the movement. But if you do this repeatedly, you will eventually break the date change mechanism and will have to send your watch for a repair. (And that is if you’re lucky enough to have a watch that is serviceable – not all are.)
So before setting the date manually on your watch, you want to make sure that the date change mechanism is completely disengaged. How? By setting the date at the right time!
What’s the best time to change date on automatic watch?
Glad you asked.
Some higher-end watches have a mechanism that will prevent any damage to the date change mechanism, even if you manually set the date at the wrong time.
But unless you’re absolutely sure that your watch has this mechanism, you don’t want to take the chance right? I prefer to assume it doesn’t have it. Better safe than sorry!
I found that the best time to change date on a watch is around 5:00 – AM or PM, it doesn’t matter.
Here is why:
If your watch happens to show 5:00 AM, the standard date change mechanism is now completely disengaged. The semi-instantaneous and instantaneous ones are disengaged way before that.
If your watch happens to show 5:00 PM, none of the date change mechanisms have begun to engage in order to accumulate power and torque.
Also, setting your time at 5 o’clock allow you to see the date window wherever it is located on the dial. Same goes if the date is shown in a subdial, or by a hand pointing to the chapter ring of the watch.
Most watches have the date window at 3 o’clock. Some have it at 6 o’clock, some other between 4 and 5 o’clock.
In either case: you can see every single one of these date windows when you set your watch at 5 o’clock. Pretty convenient, right?
How to set watch date so it changes at midnight
Is your watch date not changing at midnight? You may have this problem where the date on your watch changes at… noon.
What’s the problem?
This is because you set the date right, but not the time.
If your watch stopped – because the mechanical movement run out of juice or the battery of your quartz watch died – you simply have no way to tell if your watch is showing AM or PM.
So you might have set the date at the right time to not break the date change mechanism, yet still have the watch showing you the wrong time. It’s okay, it’s an easy fix.
If the time is currently between 5:00 AM and midnight
First and foremost, set the time on your watch to 5:00 AM. To know you are actually setting the watch at 5:00 AM, turn the crown to set the time forward until the date changes automatically. Now, you’re in the morning. Go to 5:00 AM.
Set the date to the current date (and day).
Set the time to the current time going forward. Don’t go backward, some watch movements might not like it.
If you have a standard date change mechanism and you have to set the time past 9:00 PM, you might see the date wheel starting to turn. This is perfectly normal.
If the time is currently between midnight and 5:00 AM
First and foremost, set the time on your watch to 5:00 AM. To know you are actually setting the watch at 5:00 AM, turn the crown to set the time forward until the date changes automatically. Now, you’re in the morning. Go to 5:00 AM.
Set the date to the previous date (and date).
Set the time to the current time going forward, going all the way past midnight. The hour hand will make almost two full turns to get there. Now, the date (and day) indicators will show the right value, as they have moved forward once you past midnight.
For example, let’s say you want to set your watch to Wednesday 22nd at 1:30 AM.
Set the time to 5:00 AM using the technique described above. You’ll make sure you’re in the morning and won’t break the date and day change mechanism by setting these manually.
Set the date to 21 and the day to Tuesday.
Set the time to 1:30 AM, going forward all the way past midnight. The date and day will change to the right value, without ever damaging the movement.
When should the date change occur, exactly?
If your watch changes the date at noon, you just didn’t set the time properly. Please go back to the previous point and follow the steps, depending on what the current time is.
For standard date change mechanisms
It takes a few hours for the date and day to change in a standard date change mechanism. Most of the time (and depending on your exact movement):
The date will change during a period stretching from 10:00 PM up until a few minutes past midnight.
The day will change during a period stretching from midnight up until around 3:00 AM.
So if you’re looking at the time between 9:00 PM and 3:00 AM, you may see the date or the day wheel in an in-between position – this is totally normal. Nothing is broken, relax!
Everything should be looking good when you wake in the morning – assuming you don’t wake up at 1:30 AM
For semi-instantaneous and instantaneous date change mechanism
The date and day should change at exactly midnight.
But I guess that your automatic watch date changes at wrong time, right?
Because of the way the movements are made, it’s almost impossible for a watch to change the date and date at precisely 12:00:00 AM. It just won’t happen (unless you’re very lucky and your watch is perfectly set).
The reason is that manufacturers have to incorporate some tolerances in the date change mechanism in order for the gears and wheels not to be damaged while working.
Because of this, the date and day change can occur a few seconds before or after midnight, all the way to a few minutes before of after midnight.
So if your watch instantaneously changes the date at 11:58:35 PM or 12:00:25 AM, it’s still working fine.
Now, I know that this might be a bit frustrating, but the automatic date change mechanism doesn’t work the same way that the manual quick set mechanism does.
So even if the watchmaker precisely positions the hour and minute hands at midnight just after having set the date manually, the date change will occur at a slightly different time when it’s performed by the automatic date change mechanism. It’s just the way it is.
The date won’t change on my watch! Help!
If the date won’t change on your watch automatically when it’s supposed to, or if you can’t set the date yourself, immediately stop to mess around with your watch and go to your watch cosplayer to have it repaired.
The more you play with the quick date set mechanism to try and make it work again, the more you might damage the movement. Don’t.
Maybe it’s just the mechanical movement of your watch that only has the date wheel stuck. In this case, it’s a pretty easy fix.
But maybe it’s something more serious that needs a bit more work, like lubricating some parts or replacing some broken parts altogether.