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Chapter5.Boost.Date_Time

Jeff Garland

Subject to the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)

Table of Contents

Conceptual
Motivation
Domain Concepts
Design Concepts
General Usage Examples
Gregorian
Date
Date Duration (aka Days)
Date Period
Date Iterators
Date Generators/Algorithms
Gregorian Calendar
Posix Time
Ptime
Time Duration
Time Period
Time Iterators
Local Time
Time Zone (abstract)
Posix Time Zone
Time Zone Database
Custom Time Zone
Local Date Time
Local Time Period
Date Time Input/Output
Format Flags
Date Facet
Date Input Facet
Time Facet
Time Input Facet
Date Time Formatter/Parser Objects
Date Time IO Tutorial
Serialization
Details
Calculations
Design Goals
Tradeoffs: Stability, Predictability, and Approximations
Terminology
References
Build-Compiler Information
Tests
Change History
Acknowledgements
Examples
Dates as Strings
Days Alive
Days Between New Years
Last Day of the Months
Localization Demonstration
Date Period Calculations
Print Holidays
Print Month
Month Adding
Time Math
Print Hours
Local to UTC Conversion
Time Periods
Simple Time Zones
Daylight Savings Calc Rules
Flight Time Example
Seconds Since Epoch
Library Reference
Date Time Reference
Gregorian Reference
Posix Time Reference
Local Time Reference

Introduction

A set of date-time libraries based on generic programming concepts.

This documentation is also available in PDF format. It can be found at:

http://www.crystalclearsoftware.com/libraries/date_time/date_time.pdf

In addition, a full doxygen reference can be found at:

http://www.crystalclearsoftware.com/libraries/date_time/ref_guide/index.html

The most current version of the documentation can be found at:

http://www.crystalclearsoftware.com/libraries/date_time/index.html

Conceptual

Motivation

The motivation for this library comes from working with and helping build several date-time libraries on several projects. Date-time libraries provide fundamental infrastructure for most development projects. However, most of them have limitations in their ability to calculate, format, convert, or perform some other functionality. For example, most libraries do not correctly handle leap seconds, provide concepts such as infinity, or provide the ability to use high resolution or network time sources. These libraries also tend to be rigid in their representation of dates and times. Thus customized policies for a project or subproject are not possible.

Programming with dates and times should be almost as simple and natural as programming with strings and integers. Applications with lots of temporal logic can be radically simplified by having a robust set of operators and calculation capabilities. Classes should provide the ability to compare dates and times, add lengths or time durations, retrieve dates and times from clocks, and work naturally with date and time intervals.

Another motivation for development of the library was to apply modern C++ library design techniques to the date-time domain. Really to build a framework for the construction of building temporal types. For example, by providing iterators and traits classes to control fundamental properties of the library. To the authors knowledge this library is the only substantial attempt to apply modern C++ to a date-time library.

Domain Concepts

The date time domain is rich in terminology and problems. The following is a brief introduction to the concepts you will find reflected in the library.

The library supports 3 basic temporal types:

  • Time Point -- Specifier for a location in the time continuum.
  • Time Duration -- A length of time unattached to any point on the time continuum.
  • Time Interval -- A duration of time attached to a specific point in the time continuum. Also known as a time period.

Each of these temporal types has a Resolution which is defined by the smallest representable duration. A Time system provides all these categories of temporal types as well as the rules for labeling and calculating with time points. Calendar Systems are simply time systems with a maximum resolution of one day. The Gregorian system is the most widely used calendar system today (the ISO system is basically a derivative of this). However, there are many other calendar systems as well. UTC (Coordinated Universal Time) is a widely used civil time system. UTC is adjusted for earth rotation at longitude 0 by the use of leap seconds (This is not predictable, only as necessary). Most local time systems are based on UTC but are also adjusted for earth rotation so that daylight hours are similar everywhere. In addition, some local times include daylight savings time (DST) adjustments to shift the daylight hours during the summer.

A Clock Device is software component (tied to some hardware) that provides the current date or time with respect to a time system. A clock can measure the current time to a known resolution which may be higher or lower than a particular time representation.

The library provides support for calculating with dates and times. However, time calculations are not quite the same as calculating with integers. If you are serious about the accuracy of your time calculations need to read about Stability, Predictability, and Approximations.

Design Concepts

A large part of the genesis of this library has been the observation that few date-time libraries are built in a fashion that allows customization and extension. A typical example, the calendar logic is built directly into the date class. Or the clock retrieval functions are built directly into the time class. These design decisions usually make it impossible to extend or change the library behavior. At a more fundamental level, there are usually assumptions about the resolution of time representation or the gregorian calendar.

Often times, the result is that a project must settle for a less than complete library because of a requirement for high resolution time representation or other assumptions that do not match the implementation of the library. This is extremely unfortunate because development of a library of this sort is far from a trivial task.

While the design is far from perfect the current design is far more flexible than any date-time library the author is aware of. It is expected that the various aspects of extensibility will be better documented in future versions. Information about the design goals of the library is summarized here.

Last revised: June 12, 2005 at 21:53:10 +0100


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