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Precise time for setting a watch.
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The habits that keep a mechanical watch accurate and its date mechanism intact.
Unscrew the crown if it screws down, then pull it all the way out as your seconds hand reaches the 12 o'clock position at the top of the dial. On most modern movements it stops there. Set the hands to the next full minute, watch the seconds hand on the watch above, and push the crown home the instant it reaches the top of its dial. Screw the crown back down.
For hours around midnight the date mechanism is already engaged, and using the quick-set date then can jam it or damage it. The safe habit for any watch: turn the hands until the hour hand points straight down at the 6 o'clock position before touching the date. Manufacturers state the risky hours for their own movements; the widest of the published windows below, , is what this page warns about.
| Movement | Avoid setting the date | Stated in |
|---|
If your movement isn't listed, its manual is the authority. Each row above was checked against the manufacturer's own document; brands whose guides could not be verified are left out rather than guessed.
Set the minute hand a few minutes early, then advance it clockwise onto the mark. That takes up the play in the gear train, so the hand settles exactly where you left it instead of drifting late.
A stopped automatic needs power before it will keep time. Give the crown 20 to 40 turns in the winding position, then set the time. Wearing it will do the rest.
Chronometer-grade movements are rated between −4 and +6 seconds a day by COSC; Rolex's Superlative standard is ±2, and METAS certification allows 0 to +5. Many entry-level automatics run wider. Checking against this page once a week keeps a good watch within a minute.
Turn on the GMT hand under the watch and choose a second zone. The red hand points to that zone's hour on the 24-hour scale inside the dial: 24 at the top is midnight, 12 at the bottom is noon. Unlike a mechanical GMT watch, it also shows half-hour and 45-minute zones correctly.
Your computer's clock is not used. When the page opens it asks the nearest AWS edge server for the time several times in quick succession, keeps the answer with the shortest round trip, and runs the watch from that moment on using your browser's monotonic clock. The edge servers are kept on time by Amazon's satellite-connected atomic reference clocks.
The uncertainty is about half the round trip to that edge, a few hundredths of a second on a good connection and far tighter than a human hand on a crown. The page checks again every half hour if you leave it open, and whenever you come back to the tab or your computer wakes up.
If no server can be reached, the page falls back to this device's clock and says so plainly. Time zones and daylight saving rules come from your browser's built-in database, so they stay right even for places whose rules changed recently.