Chapter 10: Times and Dates

Within a program, we may be interested in two kinds of time:

• Real time: This is the time as measured either from some standard point (calendar time) or from some fixed point (typically the start) in the life of a process (elapsed or wall clock time). Obtaining the calendar time is useful to programs that, for example, timestamp database records or files. Measuring elapsed time is useful for a program that takes periodic actions or makes regular measurements from some external input device.

• Process time: This is the amount of CPU time used by a process. Measuring process time is useful for checking or optimizing the performance of a program or algorithm.

Most computer architectures have a built-in hardware clock that enables the kernel to measure real and process time. In this chapter, we look at system calls for dealing with both sorts of time, and library functions for converting between human-readable and internal representations of time. Since human-readable representations of time are dependent on the geographical location and on linguistic and cultural conventions, discussion of these representations leads us into an investigation of timezones and locales.

10 Times and Dates
10.1 Calendar Time
10.2 Time-Conversion Functions
        10.2.1 Converting time_t to Printable Form
        10.2.2 Converting Between time_t and Broken-down Time
        10.2.3 Converting Between Broken-down Time and Printable Form
10.3 Timezones
10.4 Locales
10.5 Updating the System Clock
10.6 The Software Clock (Jiffies)
10.7 Process Time
10.8 Summary
10.9 Exercises