Is this clock directly synchronized to NIST-F4?

No. NIST-F4 is a laboratory cesium-fountain frequency standard, not a public browser feed. This tool reads the device clock, which an operating system normally synchronizes over the network, but the browser cannot certify its offset or traceability.

Do milliseconds mean the clock is accurate to one millisecond?

No. Milliseconds describe display resolution, not verified accuracy. Device-clock error, operating-system synchronization, network delay, browser scheduling and display refresh can all be larger.

What does NIST-F4 do?

NIST-F4 measures the cesium-133 transition with a fountain design and serves as a primary frequency standard. Its rate measurements help calibrate clocks used to realize UTC, including UTC(NIST).

Why can the clock not show a leap second?

JavaScript Date uses Unix-style time and cannot represent 23:59:60. The display therefore follows the device's time model during an inserted leap second.

Atomic Clock

View a precise device-clock display for UTC, local time and major world zones with milliseconds, ISO time, Unix time and a truthful NIST-F4 reference.

Initializing Device Clock

Clock Source

This display reads your device clock. Most operating systems synchronize in the background, but this browser cannot verify the clock's accuracy or connect directly to NIST's NTP service.

Milliseconds show display resolution, not guaranteed timing accuracy. JavaScript clocks also cannot display a leap second as 23:59:60.

Open Official U.S. Time

NIST-F4 Reference

NIST-F4 is a cesium fountain primary frequency standard. It calibrates the rate of clocks that help realize UTC; it is not a public time-of-day feed.

Atom: Cesium-133
Evaluated Uncertainty: 2.2 × 10⁻¹⁶
Equivalent Scale: Less Than 1 Second In 100 Million Years

Learn About Cesium Fountain Clocks

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What Is the Atomic Clock?

The Atomic Clock is a full-screen-capable time instrument inspired by the measurement discipline of NIST-F4. It displays your device's current time in UTC, local time or a selected world zone, with optional milliseconds, a 60-second ruler, the current ISO 8601 timestamp, Unix time, Julian Date, Modified Julian Date and day of year. A cesium-fountain rail references how NIST-F4 measures the second without pretending that a web page is the laboratory clock itself.

How It Works

Choose UTC, Device Local Time or another zone, then select a 12-hour or 24-hour display and whether to show milliseconds. Copy the current time or open the display full-screen. The source line always identifies this as the device clock; use the link to time.gov when you need NIST's official network-corrected U.S. time display.

When to Use It

Use it as a clean UTC reference, a full-screen room clock, a production or broadcast clock, a timestamp source, or a quick comparison between UTC and local time. The scientific fields are useful for astronomy, logging, software testing and operations work.

Frequently Asked Questions

Is this clock directly synchronized to NIST-F4?: No. NIST-F4 is a laboratory cesium-fountain frequency standard, not a public browser feed. This tool reads the device clock, which an operating system normally synchronizes over the network, but the browser cannot certify its offset or traceability.
Do milliseconds mean the clock is accurate to one millisecond?: No. Milliseconds describe display resolution, not verified accuracy. Device-clock error, operating-system synchronization, network delay, browser scheduling and display refresh can all be larger.
What does NIST-F4 do?: NIST-F4 measures the cesium-133 transition with a fountain design and serves as a primary frequency standard. Its rate measurements help calibrate clocks used to realize UTC, including UTC(NIST).
Why can the clock not show a leap second?: JavaScript Date uses Unix-style time and cannot represent 23:59:60. The display therefore follows the device's time model during an inserted leap second.

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