What is NTP (Network Time Protocol)?
NTP, the Network Time Protocol, is how computers set their clocks over the internet. Your device asks a time server what time it is, measures how long the answer took to arrive, and corrects its own clock, usually to within a few hundredths of a second.
How it works
Your computer sends a small request to a time server and notes the moment it left. The server stamps the moment it received the request and the moment it replied, and your computer notes when the reply arrived. From those four timestamps it works out both the network delay and how far its own clock is off, then nudges itself into line. It repeats this regularly, because every computer clock drifts.
Stratum: how far from the source
NTP organizes servers in layers called strata. Stratum 0 is a reference clock itself: an atomic clock or a GPS receiver. Stratum 1 servers are connected directly to one; stratum 2 servers get their time from stratum 1, and so on down. Most home computers sit a few layers down, which is still plenty accurate.
How accurate is it?
Over the public internet NTP typically keeps a computer within tens of milliseconds of true time; on a fast local network it can do better than a millisecond. The limit is mostly the network: delays that are uneven in each direction are hard to correct.
Who runs the servers?
Windows uses time.windows.com by default, Apple devices time.apple.com, and many Linux systems the volunteer pool.ntp.org project. National time labs such as NIST in the United States also run public servers traceable to their atomic clocks.
A short history
NTP was created by David L. Mills at the University of Delaware and first standardized in the 1980s, making it one of the oldest internet protocols still in everyday use. The current version, NTPv4, is defined in RFC 5905 (2010). A simplified form, SNTP, is common in small devices.
How this site checks your clock
Browsers can't speak NTP directly, so this site does something similar over the web: it asks Cloudflare's servers for the time, measures the round trip, and tells you how far your device is off. See how we get the time or check your clock on the exact time page.
Questions people ask
What does NTP stand for?
Network Time Protocol.
What port does NTP use?
UDP port 123.
Is NTP the same as atomic time?
No, but the best NTP servers get their time from atomic clocks or GPS, so NTP passes atomic-clock time on to ordinary computers.
Sources
Last updated October 21, 2026. Menu names can differ slightly between versions. Found a step that's changed? Tell us.