A curated list of resources about time: definitions, standards, time zones, calendars, atomic clocks, synchronization protocols, software, hardware, and more.
- Definitions & Standards
- Leap Seconds
- Epochs, Eras & Rollovers
- Time Zones
- National Metrology Institutes
- Network Time Protocol (NTP)
- Precision Time Protocol (PTP)
- Other Synchronization Protocols
- Time Synchronization Hardware
- Time-as-a-Service Providers
- Software Libraries
- Security Research
- Incidents & Post-mortems
- Operational Guides & Best Practices
- Books, Papers & Long-form Reading
- Talks
- Communities & Projects
- UTC (Coordinated Universal Time) - Maintained by the BIPM from a weighted average of atomic clocks worldwide.
- ISO 8601 - Date and time representation standard.
- RFC 3339 - Internet date/time format profile of ISO 8601.
- Time standard - Wikipedia overview of the timescales in current use.
- GPS time - UTC as of January 1980, no leap seconds since, currently 18 s ahead.
- IERS - Publishes DUT1 and announces leap seconds in Bulletin C.
- UT1 - The astronomical scale derived from the Earth's rotation.
- Leap second - Wikipedia overview of why and how leap seconds are inserted.
- IERS Bulletin C leap second data - Historical record of leap second announcements.
- There's a Growing Crisis Over the Leap Second - Nature on the 2022 vote to abandon the leap second by 2035.
- Leaping seconds - Tony Finch (tzdata/NTP Pool contributor) on the mechanics and politics of leap seconds.
- The Leap Second Behaviour of NTP Servers - TMA 2016 measurement study of how real NTP servers handle leap seconds.
- It's Time to Leave the Leap Second in the Past - Meta engineering's case for smearing leap seconds instead of stepping them.
- CGPM Resolution 4 (2022) - The primary source for ending leap seconds by 2035.
- GPS week number rollover - The 10-bit week number wraps every 1024 weeks; receiver firmware pivot dates decide when it actually bites.
- NTP era and the 2036 rollover - Mills on NTP's 136-year eras, which wrap two years before the POSIX problem.
- The 2038 problem in the Linux kernel - LWN
on giving 32-bit architectures a 64-bit
time_t. - Year 2038 problem - 32-bit
signed
time_truns out on 2038-01-19T03:14:07Z.
- IANA Time Zones Database (tzdata) - The
canonical source of timezone rules used by nearly every OS and language.
- Sources and Links - Community-contributed content contained in the time zone database.
- tz database background (theory.html) - How timezone rules and historical edge cases are decided.
- Working with Time Zones - W3C note on correctly handling time zones and offsets on the web.
- METAS (Swiss Federal Institute of Metrology) - Swiss national time and frequency standard.
- NIST Time and Frequency Division - US civilian time standard, including the NIST-F2 cesium fountain clock.
- PTB (Physikalisch-Technische Bundesanstalt) - German national metrology institute, atomic clock research.
- RFC 958 - Network Time Protocol, the 1985 original.
- RFC 1059 - NTP Version 1.
- RFC 1119 - NTP Version 2.
- RFC 1305 - NTP Version 3.
- RFC 5705 - Keying Material Exporters for TLS, the mechanism NTS-KE derives its keys from.
- RFC 5905 - Network Time Protocol Version 4: Protocol and Algorithms Specification.
- RFC 5906 - NTP Autokey, the broken predecessor to NTS.
- RFC 8573 - Message Authentication Codes for the Network Time Protocol.
- RFC 8915 - Network Time Security for the Network Time Protocol.
- RFC 9109 - Network Time Protocol Version 4: Port Randomization.
- RFC 9769 - NTP Interleaved Modes.
- Network Time Protocol Version 5 (draft)
ntpd- Chrony - Modern NTP/PTP client and server;
hwtimestamp, interleaved mode,prefer/requiredirectives. - NTPsec - Security-hardened fork of the reference NTP implementation.
ntpd-rs- Rust NTP implementation with NTS support.- facebook/time - Meta's Go NTP/PTP implementation, used by its own time infrastructure.
- chrony_exporter - Prometheus exporter for chrony tracking stats.
- meinberg_ltos_exporter - Prometheus exporter for Meinberg LANTIME (LTOS) appliances.
- ntpmon - NTP/chrony health monitor with Nagios checks and Prometheus/Telegraf output.
TIME.md- Notes fromnode_exporteron measuring clock sync/drift correctly.- Understanding
reach- How NTP's 8-bit reach shift register reports poll success/failure.
- IEEE 1588 - Precision Time Protocol for sub-microsecond synchronization.
- ITU-T G.8275.1/.2 - Telecom profiles for PTP (phase/time distribution).
ptp4l/ linuxptp - Linux PTP implementation, often paired withphc2sys.
- DCF77 - German longwave time signal; MSF (UK) and WWVB (US) are the equivalents.
- Roughtime - Signed responses, so a client can prove a server lied.
- White Rabbit - CERN's PTP profile for sub-nanosecond synchronization over fibre.
- Meinberg LANTIME - GPS/GNSS timeservers, Stratum 1 appliances.
- Microchip/Symmetricom TimeProvider - Grandmaster clocks for telecom-grade PTP.
- u-blox ZED-F9T - Timing-focused GNSS module.
- AWS Time Sync Service
- Cloudflare Time Services
- Google Public NTP
- Meta's open-source Time Appliance - Meta's Stratum 1 hardware/firmware, released as an OCP Time Appliance.
- chrono (Rust) - Date and time library for Rust.
- java.time - Modern Java date/time API (JSR-310).
- Python
zoneinfo- Standard library IANA timezone support. - Temporal (JS) - TC39 proposal aiming
to fix JavaScript's
Dateproblems.
- Sharon Goldberg's NTP security publications - Research on NTP protocol vulnerabilities, including off-path and authenticated-broadcast attacks.
- The Security of NTP's Datagram Protocol - Malhotra & Goldberg, FC'17: formal analysis of NTP's on/off-path threat model.
- Preventing (Network) Time Travel with Chronos - Deutsch et al., NDSS'18: a client-side defense against time-shifting attacks.
- Taming the 800 Pound Gorilla: The Rise and Decline of NTP DDoS Attacks - Czyz et al., IMC'14: measurement study of the 2014 NTP amplification wave.
- On Borrowed Time: Measurement-Informed Understanding of the NTP Pool's Robustness to Monopoly Attacks - Beverly & Rye, NDSS'26: shows a handful of malicious servers could capture most NTP Pool traffic in many countries.
- Network Time Security (NTS): Updated Security for NTP - Meinberg's overview of what NTS actually fixes.
- NTS Whitepaper - Netnod's introduction to Network Time Security concepts and deployment.
- NTP amplification DDoS attack - Why
mode 6
monlistmust not be reachable.
- D-Link settles time server dispute - How the D-Link case ended.
- Flawed Routers Flood University of Wisconsin Internet Time Server - Dave Plonka on 707,147 Netgear routers with a hardcoded server and a one-second poll interval.
- NTP server misuse and abuse - Catalogue of hardcoded-server incidents from Tardis (2002) to Yandex (2024).
- Open Letter to D-Link about their NTP vandalism - Poul-Henning Kamp on 43 hardcoded stratum 1 servers and the transit bill he was sent for them.
- Telstra outage caused by failure to prioritise known vulnerabilities - The review's finding that network timing was not treated as a critical capability.
- Telstra: what we've learned from our July outage - A timing node returned from maintenance with the wrong date, disrupting service for ~8.8 million customers.
- What would happen if you set your clock five years ahead - The 2016 NANOG thread listing what breaks.
- Best Practices for NTP Services - Carnegie Mellon SEI on running NTP securely and reliably.
- Best Practices for Connecting to NTP Servers - RIPE Labs on picking and configuring upstream time sources.
- Time Synchronization Errors Caused by Network Asymmetries - Meinberg KB on why asymmetric network paths degrade NTP accuracy.
- Time Synchronization in Virtual Machines - Meinberg KB on the pitfalls of keeping time inside VMs.
- Real Life NTP - Practical walkthrough of configuring a resilient NTP hierarchy.
- The School for Sysadmins Who Can't Timesync Good - Five-part practical series on diagnosing and fixing NTP in the real world.
- A Brief History of NTP Time: Confessions of an Internet Timekeeper - David L. Mills on the origins and evolution of NTP.
- Computer Network Time Synchronization - Mills' book on NTP's design and algorithms.
- Mastering Time Protocols - Philippe Faucon on time protocols across aerospace and network interfaces.
- An Introduction to Modern Timekeeping and Time Transfer - Banerjee and Matsakis, Springer 2023, ISBN 978-3-031-30779-9.
- Maintaining the Time in a Distributed System - Marzullo & Owicki, PODC'83: the algorithm behind NTP's clock selection.
- A Survey of the NTP Network - Nelson Minar, 1999: an early Internet-wide crawl of NTP topology and health.
- The Thorny Problem of Keeping the Internet's Time - The New Yorker profiles the NTP Pool and the people who keep it running.
- Weberblog.net NTP series - Multi-part deep dive into NTP internals and packet analysis.
- "Leaping Seconds" - The Internet Protocol Journal, vol. 15, no. 3 (Sep 2012).
- "Network Time Protocol" - The Internet Protocol Journal, vol. 15, no. 4 (Dec 2012).
- "UTC is Enough for Everyone, Right?" - Zach Holman's talk on the countless ways time zone handling goes wrong.
- David L. Mills on NTP - Mills explaining the selection, clustering and discipline algorithms himself.
Contributions welcome - see CONTRIBUTING.md.