# Your laptop was off all week. How does it still know the time?

- Series: [How Computers Work](https://miu.fm/howcomputerswork.md)
- Premieres: 2026-11-07
- Page: https://miu.fm/howcomputerswork/your-laptop-was-off-all-week-how-does-it-still-know-the-time

## Story

Your laptop was off all week. How does it still know the time?

It never stopped counting. Inside there's a small clock that keeps running on a trickle of battery while the rest of the machine is off. A laptop chip's own datasheet says its clock "operates on a 32.768 kHz oscillating source and a 1.5 V battery".

That source is a sliver of quartz cut like a tiny tuning fork. Squeeze quartz and it makes a voltage; put a voltage on it and it bends. The Curie brothers found that in 1880. So a circuit keeps nudging the fork in time with its own ring, and it rings 32,768 times a second.

Why that number? Halve it fifteen times and you get exactly one. Fifteen switches, each flipping half as often as the one before, turn the ring into one tick a second. Horton and Marrison built the first quartz clock at Bell Labs; it was described in October 1927.

It isn't perfect. One maker's crystal like this is allowed 20 parts in a million, up to 1.73 seconds a day, and runs slower in the cold: at 0 °C its own curve gives about 16 seconds a week (our arithmetic). So once the laptop is online it asks a time server and corrects itself. The protocol's own standard says within a few tens of milliseconds.

The fork, the switches and the laptop are our own simplified drawings.

Which clock in your home is always wrong?

Sources: Intel, Core Processor (Series 3) Datasheet, Real Time Clock (2026); Rakon, QETF26 tuning fork crystal datasheet; W. A. Marrison, The Evolution of the Quartz Crystal Clock, Bell System Technical Journal, 1948; IETF RFC 5905, Network Time Protocol Version 4 (2010); our own arithmetic and simulation.
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