What is NTP, and why does accurate time matter so much?
The Network Time Protocol — the mechanism by which almost every connected device keeps its clock correct — and the reason it matters is that a surprising amount of infrastructure fails outright when clocks disagree, rather than degrading gracefully.
How it works. A client asks a server for the time and records four timestamps: when it sent the request, when the server received it, when the server replied, and when the reply arrived. From these it can calculate both the offset and the round-trip delay, and correct for the network transit time rather than naively accepting the value.
It queries several servers, discards outliers, and disciplines the local clock gradually — slewing rather than stepping, because a clock jumping backwards breaks anything that assumes time moves forwards.
Stratum describes distance from a reference: stratum 0 is the physical source (GPS, an atomic clock), stratum 1 is a server directly attached to one, and each further hop increments. Lower is not automatically better in practice; network path quality matters as much.
What breaks when time is wrong:
TLS certificates. Validity is a time window, so a device with the wrong date rejects valid certificates — this is the single most common symptom, and it presents as "the internet is broken" rather than as a clock problem.
Authentication. Kerberos, TOTP two-factor codes and signed API requests all reject stale timestamps within a tolerance of minutes or seconds.
Logs and forensics. Correlating events across systems is impossible if their clocks disagree.
Databases and distributed systems, where ordering depends on timestamps.
Scheduled jobs, which can run twice or not at all.
Leap seconds have been the recurring hazard — an inserted second has repeatedly caused outages — which is why many large operators smear the adjustment across hours instead, and why the decision has been taken to discontinue them.
Security. Unauthenticated NTP has been abused for amplification, and NTS adds authentication.