Systems Programming/ C and UNIX

Systems Programming/ C and UNIX

A. Fischer ? CSCI 4547/6647 What is Time?

November 6, 2017

A. Fischer ? CSCI 4547/6647[1ex]What is TimSey?stems Programming ? Lecture 8. . . 1/24

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Outline

1 What is time?

2 Unix Time Representations of Time Time Functions

A. Fischer ? CSCI 4547/6647[1ex]What is TimSey?stems Programming ? Lecture 8. . . 2/24

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What is time?

What is time?

Real Time and Elapsed Time Universal Time Time Accuracy

A. Fischer ? CSCI 4547/6647[1ex]What is TimSey?stems Programming ? Lecture 8. . . 3/24

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What is time?

Real Time and Elapsed Time

We use the word "time" in two ways: When: at what point in the history of the universe did an event take place. What time is it now? I will use "time" or "clock time" or "real time" to talk about when. How long: how many time units elapsed between the beginning and the end of an event? I will call this "elapsed time".

This seems like a simple subject but it is truly NOT simple.

A. Fischer ? CSCI 4547/6647[1ex]What is TimSey?stems Programming ? Lecture 8. . . 4/24

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Universal Time

What is time?

We have to measure a physical phenomenon, like time, before we can talk about it, store it, or use it.

The world has an official time standard, maintained by a standards committee. It is based on astronomical observations and an atomic clock:

UT1 is one form of Universal time, established by measuring the Earth's movement relative to a distant quasar. It is the same if measured anywhere on earth. The problem is that Earth's rotation is slightly irregular. UTC, or Coordinated Universal Time is measured using a completely regular atomic clock. It approximates UT1 and is the basis for world-wide civil time. This is kept synchronized with UT1 by adding leap-seconds to the UTC clock when the drift reaches .9 seconds.

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