Core concepts in Noda Time

This is a companion page to the "core types quick reference", and "choosing between types" pages, describing the fundamental concepts in Noda Time.

One of the benefits of Noda Time over the Base Class Library (BCL) time support is the representation of different concepts with different types. The downside of this is that there are more types to learn about, and you're forced to make decisions between subtly different concepts where in the BCL you could just use DateTime and hope you were doing the right thing.

By forcing you to think a little bit more upfront, we're hoping you'll gain a greater understanding not only of how your code works but sometimes what data you're trying to represent anyway - you should expect the use of Noda Time to clarify your thinking around date and time data for your whole project.

This document introduces the core concepts, but in order to avoid it being too overwhelming, we won't go into the fine details. See individual pages (particularly the "choosing between types" page) and the API documentation for more information.

"Local" and "global" (or "absolute") types

Many of the concepts can be considered as either local or global. These terms are fairly common within other APIs and documentation, but they can be confusing to start with. (Global values are also called absolute values in some writing, although we don't use this name in Noda Time.)

The key difference is that people all around the world will agree on a global value simultaneously, whereas they may all see different local values for the same global value, due to time zones. A local value has no associated time zone in Noda Time; in particular it is not just "a date and time fixed to the local time of the computer running the code" as a DateTime with a Kind of Local is interpreted in .NET.

Internally, Noda Time has a concept of a LocalInstant which is a local value without reference to any particular calendar system, but this is currently not exposed. Similarly, LocalTime does not refer to a calendar system - we assume that all calendars are based around solar days 24 hours in length (give or take daylight saving changes). However, the LocalDate and LocalDateTime types do both refer to calendar systems... see the later section on calendars for more information.

The global time line, and Instant

Noda Time assumes a single non-relativistic time line: global time progresses linearly, and everyone in the universe would agree on the global concept of "now". Time is measured with a granularity of ticks where there are 10,000 ticks in a millisecond. This is the same concept of a tick which is used in DateTime and TimeSpan in .NET, although a Stopwatch tick in .NET can vary based on the timer used.

The "zero point" used everywhere in Noda Time is the Unix epoch: midnight at the start of January 1st 1970, UTC. (UTC itself did not exist in 1970, but that's another matter.) The Unix epoch happens to be a common zero point used in many other systems - but if we had used some other zero point (such a January 1st 2000 UTC) it would simply have offset the values, and changed the maximum and minimum representable values. It's just an origin.

The Noda Time Instant type represents a point on this global timeline: the number of ticks which have elapsed since the Unix epoch. The value can be negative for dates and times before 1970 of course - the range of supported dates is from around 27000 BCE to around 31000 CE in the Gregorian calendar.

An Instant has no concept of a particular time zone - it is just the number of ticks which have occurred since the Unix epoch. The fact that the Unix epoch is defined in terms of UTC is irrelevant - it could have been defined in terms of a different time zone just as easily, such as "1am on January 1st 1970 in the Europe/London time zone" (as the UK time zone was experimentally an hour ahead of UTC at the time). The tick values would remain the same.

Similarly, an Instant has no concept of a particular calendar system - it is meaningless to ask which month an instant occurs in, as the concept of a month (or year, etc) is only relevant within a particular calendar system.

Instant is a good type to use for "when something happened" - a timestamp in a log file, for example. The instant can then be interpreted in a particular time zone and calendar system, in a way which is useful to the person looking at the log.

Calendar systems

Humans break up time into units such as years, months, days, hours, minutes and so on. While time itself has no such concept, it makes life more convenient for people. Unfortunately - and this is a recurrent theme in software - humanity hasn't done very well in agreeing a single system to use. So there are multiple different calendars, including Gregorian, Julian, Coptic and Buddhist. These allow different people to talk about the same local time in different ways - the Unix epoch occurred on December 19th 1969 in the Julian calendar system, for example.

The calendar system you use is one of the few things Noda Time is willing to use a default for: unless you specify otherwise, the ISO-8601 calendar is used. If you don't know which calendar you ought to use, this is almost certainly the one for you.

In Noda Time, the CalendarSystem type handles the details of different calendar systems. Most of the methods are internal, although a few useful methods are exposed. Most of the time even if you do need to use a CalendarSystem, you can just fetch a reference to an appropriate object, and then pass it to other constructors etc as a little bundle of magic which simply does the right thing for you.

Time zones

In the most basic sense, a time zone is a mapping between UTC date/times and local date/times - or equivalently, a mapping from UTC instants to offsets, where an offset is simply the difference between UTC and local time. In Noda Time, time zones are represented by the DateTimeZone class, and offsets are represented by the Offset struct.

An offset is positive if local time is later than (ahead of) UTC, and negative if local time is earlier than (behind) UTC. For example, the offset in France is +1 hour in the winter and +2 hours in the summer; the offset in California is -8 hours in the winter and -7 hours in the summer. So at noon UTC in winter, it's 4am in California and 1pm in France.

As well as mapping any particular instant to an offset, DateTimeZone allows you to find out the name of the part of the time zone for that instant, as well as when the next or previous change occurs - usually for daylight saving changes.

Most of the time when you use a DateTimeZone you won't need worry about that - the main purpose is usually to convert between a ZonedDateTime and a LocalDateTime - where the names mean exactly what you expect them to. There's a slight twist to this: converting from an Instant or a ZonedDateTime to a LocalDateTime is unambiguous; at any point in time, all the (accurate) clocks in a particular time zone will show the same time... but the reverse isn't true. Any one local time can map to:

Noda Time makes it reasonably easy to handle these situations, but you need to work out what you want to happen. See the DateTimeZone documentation for more details and options.

There are various different sources of time zone information available, and Noda Time handles two of them: it is able to map BCL TimeZoneInfo objects using BclDateTimeZone, and the TZDB (aka Olson) database. A version of TZDB is embedded within the Noda Time distribution, and if you need a more recent one, there are instructions on how to download and use new data. We generally recommend that you isolate yourself from the provider you're using by only depending on IDateTimeZoneProvider, and injecting the appropriate provider in the normal way. "Stock" providers are available via the DateTimeZoneProviders class.

Also note that in some cases, you may not have full time zone information, but have just a local time and an offset. For example, if you're parsing the string "2012-06-26T20:41:00+01:00" that gives the information that the local time is one hour ahead of UTC - but it doesn't give any indication of what the offset would be at any other time. In this situation, you should use OffsetDateTime.

Periods and durations

There are two similar types in Noda Time used to represent "lengths of time". The simplest is Duration which is equivalent to [TimeSpan] in the BCL. This is a fixed number of ticks - it's the same length of time wherever its applied. Duration is used for arithmetic on Instant and DateTime values.

A more complex type is [Period], which is a set of values associated with different calendar-based periods: years, months, weeks, days, hours, minutes and so on. Some of these periods represent different lengths of time depending on what they're applied to - if you add "one month" to January 1st, that's going to be 31 days long. Adding the same period to February 1st will give a shorter length of time - which then depends on whether the year is a leap year or not. Periods based on smaller units (hours, minutes and so on) will always represent the same length of time, but they're still available within periods. [Period] is used for arithmetic on locally-based values (LocalDateTime, LocalDate, LocalTime).

See the arithemtic page for more information.