Hacking Time: Spoofing Atomic Clocks with Audio Harmonics

The Texture of Time

Time is a fundamental anchor of physics, social life, governance, and business. The human drive to measure it under challenging conditions has shaped history—culminating in massive prizes like the Longitude Act of 1714 for marine timekeeping. Early mechanical clocks utilizing weight-driven escapements—the internal mechanisms that release gear teeth one by one to produce a steady "tick"—began emerging in the late 13th century, but they were incredibly fragile. For everyday, robust timekeeping, people relied on deeply analog methods. People used to stick nails into candles; as the wax burned down, the nails would fall and clatter onto a metal plate to mark the passing hours.

The sand-glass (or hourglass), which first clearly appears in the historical record around 1338–1345, was a highly portable contemporary alternative. As A.J. Turner notes in his history of the instrument (check out Balmer 1978 too!), they were widely used in courts, schools, and homes, and proved perfectly suited for the ocean because they told the exact same time regardless of how violently a ship swayed. In fact, mariners would count the number of knots on a rope pulled into the sea while a small sand glass emptied, giving us the term "knot" for nautical speed. Today, incredibly precise (and secure) timing is the invisible bedrock of modern digital and Internet infrastructure.

Personally, I swear by Coordinated Universal Time (UTC). Because our work at the Internet Society (ISOC) is truly global, I rely heavily on UTC to immediately understand what time it is for our team, collaborators, and partners spread across different continents.

This desire to have the undisputed "true" time on my wall recently led me to purchase a Marathon CL030079-BK-00-NA "atomic" digital clock. The premise of these clocks is wonderfully simple to the consumer: hidden inside is a tiny antenna designed to catch a specific radio broadcast (the WWVB signal) from the National Institute of Standards and Technology (NIST) station in Fort Collins, Colorado. It listens to this broadcast and automatically syncs its display to the national atomic clock.

There is just one problem: the physics of radio wave propagation makes getting that signal on the East Coast incredibly difficult.

The East Coast Dead Zone

The WWVB transmitters in Colorado pump out 70 kilowatts of power—roughly equivalent to a major commercial AM radio station. However, the signal operates at 60 kHz, which is a massive, long radio wave. The WWVB "ground wave" physically maxes out around 600 to 1,000 miles from the transmitter; it never reaches me here in Takoma Park, Maryland. Therefore, the East Coast relies entirely on the "skywave"—the radio signal bouncing off the Earth's ionosphere.

But the ionosphere is fickle. During the day, solar UV radiation ionizes the lower "D-layer" of the atmosphere, turning it into an electromagnetic sponge that absorbs these long radio waves. At night, without the sun's radiation, this sponge layer dissipates. The radio waves can then travel higher up and bounce off the reflective "E-layer," effectively skipping across the continent to reach Maryland. This is why these clocks are stubbornly programmed to wake up and attempt a sync at 2:00 AM.

Even at night, however, the signal is thoroughly exhausted. It has to compete with the dense "noise" of modern urban life. Every Wi-Fi router, power line, and humming computer monitor creates electronic interference. The East Coast is practically a dead zone for certain kinds of low-power signals. Because of this, my Marathon clock was essentially deaf, unable to calibrate using the atomic clock broadcast signal, slowly drifting seconds and then minutes away from true UTC.

Bridging the Gap

Now, elsewhere, I have accurate time from "network time". That is, I have the gold standard of time—the Internet's Network Time Protocol (NTP)—syncing my phone and laptop to the millisecond. But my wall clock was an isolated analog island.

I needed a bridge. Enter Clock Wave, a smartphone app that essentially turns your phone into a tiny, localized WWVB transmitter. That's right, spoof the signal to calibrate!

I must admit, when I first heard about this, I was confused. How can a smartphone spoof a 60 kHz atomic radio broadcast? The audio hardware inside a phone is built for human ears, which can only hear up to about 20 kHz. To cover this range efficiently, standard audio chips (and formats like CDs) operate at a 44.1 kHz or 48 kHz sample rate. Governed by a fundamental mathematical law of digital sampling known as the Nyquist-Shannon theorem, a 48 kHz system is mathematically hard-capped. It cannot physically represent or output frequencies above 24 kHz without causing severe digital distortion known as "aliasing." It is physically impossible for a phone's digital audio pipeline to play a 60 kHz tone.

So, how does the app register as that 60 kHz WWVB signal? Answer: It cheats using physical analog distortion.

Audio Harmonics and Voice Coils

Instead of trying to play a smooth 60 kHz tone, the Clock Wave app outputs a harsh 20 kHz square wave at maximum volume. This intentionally overdrives the smartphone's physical analog amplifier, causing the circuitry to "clip" the signal. This physical distortion creates mathematical echoes called harmonics—and the 3rd harmonic of a 20 kHz tone happens to be exactly 60 kHz.

Here is the real magic: The smartphone isn't broadcasting a radio wave at all. The speaker creates sound using a "voice coil"—a tiny electromagnet. By pulsing this electromagnet with that clipped signal, it creates a rapidly fluctuating, highly localized magnetic field right next to the phone.

The clock's internal receiver—a tuned ferrite loopstick antenna—is inherently designed to resonate with magnetic fields (inductive coupling) while remaining deaf to acoustic sound waves. It ignores the loud 20 kHz audio from the speaker and "feels" only the faint 60 kHz magnetic echo, successfully decoding the time data hidden inside.

Emulating the Broadcast

Because this trick relies on a localized magnetic "near-field" rather than a radiating radio wave, it only works at extremely close range. Radio waves are designed to detach from antennas and travel outward, dropping off in strength gradually (following the inverse-square law). Near-field magnetic energy, however, clings tightly to the source and drops off violently (according to the inverse-cube law). Moving your phone just a single inch away from the clock weakens the magnetic signal so much that it becomes completely undetectable.

Here is my workflow for syncing the Marathon CL030079-BK-00-NA:

1. Open the Clock Wave app, ensure it has fetched the exact Internet time, and set it to Transmit.
2. Turn your smartphone volume up to the absolute maximum (this drives the clipping and powers the electromagnet).
3. Rest the speaker of the smartphone directly against the bezel of the wall clock to overcome the near-field drop-off.
4. Press the Sync and Wave buttons on the back of the clock at the same time to force it to listen.

You will see the antenna icon on the clock begin to blink. Leave the phone there until the blinking icon goes solid, which means the clock has successfully locked in and updated its LCD display to perfectly match the Internet time.

Crucial last step: Once it is synced, I immediately turn off the WWVB auto-receive function on the clock. If you leave it on, the clock will wake up at 2:00 AM, try to listen for Colorado, catch a bunch of weird static from the East Coast dead zone, and occasionally you will be unpleasantly surprised by it displaying a wildly incorrect time.

The Analog Thread

We tend to think of modern time—UTC, NTP, digital synchronization—as weightless data. But getting that precise time into an isolated wall clock required bypassing the digital realm entirely. It required intentionally distorting an amplifier to pulse a microscopic copper coil 20,000 times a second, casting a localized magnetic field.

The 14th-century sailor relied on the physical flow of sand because the ocean was hostile to delicate gears. I rely on the physical vibration of a voice coil because the East Coast is hostile to longwave radio signals. We've modernized how we measure time, but occasionally, we still need a deeply analog hack to pin it accurately to the wall.