Time is a funny thing. You can spend it, save it, waste it and kill it, but you can't change it and there is never any more or less of it. Everyone knows what it is and uses it every day but no one can seem to define it. In this talk I will provide a brief introduction to time, timekeeping, and the uses of time information, especially in scientific and technical areas. http://www.nycbug.org/index.cgi?action=view&id=10349

1. A Brief Introduction
to Timekeeping
Steven Kreuzer
steven@kreuzer.cx
NYCBUG
July 2014

2. What is Time?
• It is pretty tricky to define in a way that is not
circular
• Simply a matter of counting cycles or units of time

3. What is a Clock?
• A clock consists of an oscillator and a counter
• The function of the oscillator is to establish a
repeatable interval of time
• By counting those intervals it is possible to create a
time scale

4. Greenwich Mean Time
• Mainly used by astronomers so it was based on
measurements of true earth rotation
• Rise of railroads caused the need for accurate,
uniform time leading to the creation of time zones
and standard time
• Between 1848 to 1972, all of the major countries
adopted time zones based on GMT

5. Time Zones
• 24 time zones, each differing from the next by 1
hour and span 15 degrees of longitude
• Local time is determined by government so they
usually follow the countries borders
• From time to time a country will opt to change their
time zone for various reasons

6. Coordinated Universal Time
• Earth rotation varies over time so a time scale
based on this will vary over time
• UTC is based on Atomic Time (TAI) which is a time
scale where the length of a second does not vary

7. Leap Second
• To keep UTC consistent with the actual earth
rotation speed, leap seconds are inserted
• Leap seconds can be inserted and removed, but so
far they have only been inserted
• 26 leap seconds have been inserted since 1972
• The last second was inserted on June 30th 2015

8. Evolution of The Clock
• Late 1630 - Galileo observed that a pendulum took the
same time to swing through a wide arc as a narrow arc
• Mid 1650 - Christiaan Huygens introduces the
pendulum clock
• Mid 1720 - John Harrison improved the accuracy of the
pendulum clock to 1/5 second per day
• Early 1920 - William Hamilton Shortt introduces the two-
pendulum clock and squeezed the last ounce of
perfection out of mechanical clocks.

9. The Quartz Crystal
• Piezoelectricity first demonstrated by Pierre and
Jacques Curie in 1880
• When piezoelectric material is subjected to
mechanical stress, it creates an electrical charge
• Inverse is also true
• Typical drift of a quartz watch is ~1 second per day

10. Quartz Stability
• Temperature influences the operating frequency
• Frequency will slow down when the temperature
either increases or decreases

11. Improving Quartz Stability
• Analog compensation (TCXO)
• Microcontroller compensation (MCXO)
• Temperature stabilization w/ a crystal oven (OCXO)

12. Atomic Clocks
• Changes in the energy levels of atoms release
electromagnetic radiation of very specific
frequencies
• Atoms don’t wear out, change their properties over
time or have small differences due to manufacturing
imprecisions
• Accuracy better than 1 second in six million years

13. Global Positioning System
• Cesium 133 atomic clocks on-board each satellite
• Radio signals broadcast from satellites include the
exact time the signal was transmitted
• Offers a direct and accurate connection to UTC

14. Disciplining a Clock
• Provide a reference time source which is more
accurate and set the system time in periodic
intervals
• This method allows system time to drift during each
interval
• Ideal solution is to determine the system clock drift
to discipline the system time smoothly

15. Network Time Protocol
• RFC 5905 - Network Time Protocol Version 4:
Protocol and Algorithms Specification
• RFC 5906 - Network Time Protocol Version 4:
Autokey Specification
• RFC 5907 - Definitions of Managed Objects for
Network Time Protocol Version 4
• RFC 5908 - Network Time Protocol Server Option
for DHCPv6

16. Basic Features of NTP
• Enable clients across the Internet to be accurately
synchronized to UTC
• Provide most accurate time possible, based on
‘reference time’ - not just syncing to a common time
• Ignore ‘falsetickers’ - clocks it could use for reference,
but which provide an apparently wrong time
• Use previous figures to estimate current difference
between system time and reference time, in the
absence of a network connection

17. Synchronization Hierarchy
• Clients query the reference time from one or more
servers
• Servers make its own time available as reference
time for other clients
• Peers compare its system time to other peers until
all the peers finally agree about the"true" time to
synchronize to

18. Stratum Levels
• Stratum 0 - Reference Clock Source (gps, cdma)
• Stratum 1 - Primary Time Servers
• Stratum 2 - Secondary Time Servers
• Stratum 3 - Workstations

19. Order of Operations
• The client stamps the time when it sends an NTP
packet to the server
• The server stamps the time when it receives the
packet from the client
• The server stamps the time when it sends a packet
back to the client
• The client stamps the time when the NTP reply
packet is received

20. Synchronization Problems
• Two clocks hardly ever agree
• Clocks tick at different rates
• Skew is the difference between two clocks at one
point in time

21. Defining Skew and Jitter
• Clock skew is the deterministic difference in clock
arrival times
• Clock jitter is random difference in clock arrival
times
• Jitter is always bad

22. Dealing with Drift
• If the clock is running fast, make the clock run
slower until it synchronizes
• If the clock is running slow, make the clock run
faster until it synchronizes
• The clock must always be moving forward. The
illusion of time moving backwards causes all sorts
of problems

23. Time Synchronization
• Allows events to occur at proper times
• Provide proof of when events occurred or did not
occur

24. Conclusion
• Accurate time is a necessity of modern society
• Synchronized time is a integral part of an effective
network
• Ensuring accurate time is inexpensive but offers a
significant return on investment

25. –John Wheeler
“Time Is What Prevents Everything From
Happening At Once..”