Time, Dates, and Calendars

Few programming languages have direct support for times and dates, leaving such support to libraries. This is fine. However, many languages (including Java and JavaScript) have time and date support in their standard libraries which is very poor. Java's Calendar and Date classes are particularly pathetic; JavaScript's is lame but at least fully admits its deficiencies, promising nothing. Fortunately programmers can turn to third-party libraries (including the excellent Joda-Time for Java).

Here I want to convey the basic issues in representing temporal data and operations in programming, sketch out ways to make better libraries, and look at ways to include time and date support within a language.

Basic Definitions

Many programmers have an insufficient understanding of time and dates. Here is a short, and in many cases oversimplified, overview of the main concepts:

Time

The one-dimensional continuum of experiences.

Instant

A point in time. Because time is one-dimensional, an instant can be given with a single real number, once we choose an origin and define a unit. The origin (instant 0) of the one-dimensional space is called the epoch. Common units for the description and measurement of time include the second and the millisecond.

Duration

A quantity of time, for example "17343.3422 milliseconds." Note that instants are points but durations are vectors. An instant plus a duration is an instant; An instant minus an instant is a duration.

Interval

The notion of elapsed time between two particular instants. Given an interval, you can compute its duration, but not vice versa. For example, the interval between today at noon and tomorrow at noon different from the interval between yesterday noon and today noon, even though the durations of each interval are the same.

DateTime

A description of an instant in human-friendly terms, generally, but not necessarily, based on aspects of the Earth, Sun, and Moon. DateTimes are often written with many components, e.g., 2010-08-22T17:29:41-07:00 has year, month, day, hour, minute, second, and offset components, respectively.

Calendar

A framework for describing datetimes. Also called a chronology. The chronologies natively supported by Joda-Time are:

• Gregorian
• Julian
• GJ
• ISO
• Buddhist
• Coptic
• Ethiopic

There are of course, many, many others.

Era

A large division of dates within a calendar. For example, the GJ calendar contains two eras: BCE and CE (also known as BC and AD).

Partial

A fragment of a datetime, leaving parts unspecified. For example: "August 1", "14:40". These are useful to describe recurring events, such as birthdays. They generally do not map to any particular instant or interval, but rather to many.

TimeZone

An optional component of a date time that allows people to have similar associations between the time portion of a datetime and the position of the sun relative to their location on Earth. Usually represented as an offset (duration), which can vary according to local law (e.g., Daylight Saving).

Period

Similar to a duration, but expressed with human-friendly terms like months, weeks, and days. A period does not have an exact number of milliseconds; for example "2 months" can have a different number of days, depending on the time of year; and even "1 day" can have 22, 23, or 24 hours when daylight time is taken into account.

A Time Library

A good time library should maintain types for each of the above concepts, as well as a rich set of parsing and formatting routines for dates, conversions between types, and operators. Joda-Time qualifies.

The ability to construct new calendars (chronologies) is also desirable. What would such a capability look like?

TODO

Native Language Support for Time

Because the concepts of time of dates are complex, language-level support for all of the concepts introduced above can become quite cumbersome. How many reserved words or special operators can a language, especially a general purpose one, absorb? How many literal forms should be added?

TODO