# 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.

- Basic Terminology
- Calculations
- Stability, Predictability, and Approximations
- References