# Module Zone

    module Zone: sig .. end

Time-zone handling.

# User-friendly interface

    type t

The type of a time-zone.

bin_io and sexp representations of Zone.t are the name of the zone, and not the full data that is read from disk when Zone.find is called. The full Zone.t is reconstructed on the receiving/reading side by reloading the zone file from disk. Any zone name that is accepted by `find` is acceptable in the bin_io and sexp representations.

    include Identifiable.S

    val find : string -> t option

`find name` looks up a `t` by its name and returns it.

    val find_exn : string -> t

    val local : t

`local` is the machine's local timezone, as determined from the `TZ` environment variable or the `/etc/localtime` file. It is computed from the state of the process environment and on-disk tzdata database at some unspecified moment prior to its first use, so its value may be unpredictable if that state changes during program operation. Arguably, changing the timezone of a running program is a problematic operation anyway -- most people write code assuming the clock doesn't suddenly jump several hours without warning.

Note that any function using this timezone can throw an exception if the `TZ` environment variable is misconfigured or if the appropriate timezone files can't be found because of the way the box is configured. We don't sprinkle `_exn` all over all the names in this module because such misconfiguration is quite rare.

    val likely_machine_zones : string list Pervasives.ref

`likely_machine_zones` is a list of zone names that will be searched first when trying to determine the machine zone of a box. Setting this to a likely set of zones for your application will speed the very first use of the local timezone.

    val of_utc_offset : hours:int -> t

`of_utc_offset offset` returns a timezone with a static UTC offset (given in hours).

    val default_utc_offset_deprecated : t -> int

`default_utc_offset` returns the UTC offset of default regime for timezone `t` in seconds. Note: the default utc offset may not reflect the current utc offset.

    val utc : t

`utc` the UTC time zone. Included for convenience

    val abbreviation : t -> float -> string

`abbreviation zone t` returns t abbreviation name such as EDT, EST, JST of given `zone` at the time `t`. This string conversion is one-way only, and cannot reliably be turned back into a t

    val name : t -> string

`name zone` returns the name of the time zone

    val init : unit -> unit

`init ()` pre-load all available time zones from disk, this function has no effect if it is called multiple times. Time zones will otherwise be loaded at need from the disk on the first call to find/find_exn.

    val digest : t -> string option

`digest t` return the MD5 digest of the file the t was created from (if any)

    val initialized_zones : unit -> (string * t) list

`initialized_zones ()` returns a sorted list of time zone names that have been loaded from disk thus far.

    val shift_epoch_time : t -> [ `Local | `UTC ] -> float -> float

`shift_epoch_time zone [`Local | `UTC] time` Takes an epoch (aka "unix") time given either in local or in UTC (as indicated in the arguments) and shifts it according to the local time regime in force in zone. That is, given a Local epoch time it will return the corresponding UTC timestamp and vice versa. This function is low level, and is not intended to be called by most client code. Use the high level functions provided in Time instead.

    val next_clock_shift : t -> after:Time_internal.T.t -> (Time_internal.T.t * Span.t) option

Takes a `Time.t` and returns the next `Time.t` strictly after it, if any, that the time zone UTC offset changes, and by how much it does so.

    val prev_clock_shift : t -> before:Time_internal.T.t -> (Time_internal.T.t * Span.t) option

As `next_clock_shift`, but strictly *before* the given time.

    exception Unknown_zone of string

    exception Invalid_file_format of string

    module Stable: sig .. end

# Notes on time

This library replicates and extends the functionality of the standard Unix time handling functions (currently exposed in the Unix module, and indirectly through the Time module).

Things you should know before delving into the mess of time...

## Some general resources (summarized information also appears below)

        general overview   - http://www.twinsun.com/tz/tz-link.htm
        zone abbreviations - http://blogs.msdn.com/oldnewthing/archive/2008/03/07/8080060.aspx
        leap seconds       - http://en.wikipedia.org/wiki/Leap_second
        epoch time         - http://en.wikipedia.org/wiki/Unix_time
        UTC/GMT time       - http://www.apparent-wind.com/gmt-explained.html
        TAI time           - http://en.wikipedia.org/wiki/International_Atomic_Time
        Almost every possible time measurement -
          http://www.ucolick.org/~sla/leapsecs/timescales.html

## Standards for measuring time

- Epoch time/Unix time/Posix time: Defined as the number of seconds that have passed since midnight, January 1st, 1970 GMT. However, under epoch time, a day is always 86,400 seconds long, and a minute never contains more than 60 total seconds. In other words, epoch time does not take leap seconds into account properly. What a POSIX compliant system does during a leap second depends on the way in which its clock is managed. It either ignores it, replays the second, or causes a second to last longer than a second (retards the second). The important thing to remember is that however the transition is managed, all days start on an evenly divisible multiple of 86,400.
- GMT/Greenwich Mean Time/Greenwich Civil Time: The time based on the movement of the sun relative to the meridian through the Old Greenwich Observatory (0 degrees). The movement of the sun in this case is a "mean" movement of the sun to adjust for slight eccentricities in the rotation of the earth, as well as for the effect of the tilt of the earth on the visible speed of the sun across the sky at different times of the year. GMT is often used synonymously with the term UTC (see below), but may also be used to refer to the time system described here, which differs from UTC (as of 2009) by ~1 second.
- Standard Time: The time based on the adjusted (as in GMT) movement of the sun over a point on the earth that is not Greenwich. Colloquially, the time in a time zone without accounting for any form of daylight savings time.
- Wall Clock Time: The time as it appears on a clock on the wall in a given time zone. Essentially this is standard time with DST adjustments.
- TAI: International atomic time. The time based on a weighted average of the time kept by roughly 300 atomic clocks worldwide. TAI is written using the same format as normal solar (also called civil) times, but is not based on, or adjusted for the apparent solar time. Thus, as of 2009 TAI appears to be ahead of most other time systems by ~34 seconds when written out in date/time form (2004-09-17T00:00:32 TAI is 2004-09-17T00:00:00 UTC)
- UTC/Universal Coordinated Time: Often taken as just another term for GMT, UTC is actually TAI adjusted with leap seconds to keep it in line with apparent solar time. Each UTC day is not an exact number of seconds long (unlike TAI or epoch time), and every second is exactly one real second long (unlike GMT, which is based entirely on the apparent motion of the sun, meaning that seconds under GMT slowly get longer as the earth's rotation slows down). Leap seconds are determined by the rotation of the earth, which is carefully measured by the International Earth Rotation Service in Paris, France using a combination of satellite and lunar laser ranging, very long baseline interferometry, and Navstar Global Positioning System (GPS) stations. This isn't important for using UTC, but is very cool. UTC is not well defined before about 1960.
- Windows File Time: The number of 100-nanosecond intervals that have elapsed since 12:00 A.M. January 1, 1601, UTC. This is great because UTC has no meaning in 1601 (being based on atomic timekeeping technologies that didn't exist then), and also because 1601 predates the development of even reasonably accurate clocks of any sort. The reasoning behind the Windows epoch time choice is that "The Gregorian calendar operates on a 400-year cycle, and 1601 is the first year of the cycle that was active at the time Windows NT was being designed. In other words, it was chosen to make the math come out nicely." (http://blogs.msdn.com/oldnewthing/archive/2009/03/06/9461176.aspx)
- VBScript (this is my favorite): http://blogs.msdn.com/ericlippert/archive/2003/09/16/eric-s-complete-guide-to-vt-date.aspx

All of these systems start to exhibit problems as you go further back in time, partly because truly accurate timekeeping didn't make an appearance until roughly 1958, and partly because different parts of the world didn't actually have well defined time zones for a long time. If you go back far enough, you run into the switch between the Julian (old) and the Gregorian calendar, which happened at different times in history in different places in the world.

## How does a system determine what time zone it is in?

1. Check to see if the TZ environment variable is set. If it is, it can be set to one of three forms, two of which are rarely, if ever used see:

http://www.opengroup.org/onlinepubs/000095399/basedefs/xbd_chap08.html

for more information on the obscure forms. The common form represents a relative path from the base /usr/share/zoneinfo/posix, and is generally in the form of a continent or country name paired with a city name (Europe/London, America/New_York). This is used to load the specified file from disk, which contains a time zone database in zic format (man tzfile).

1. If TZ is not set, the system will try to read the file located at /etc/localtime, which must be a zic timezone database (and which is often just a symlink into /usr/share/zoneinfo/posix).
2. If /etc/localtime cannot be found, then the system is assumed to be in GMT.

It's worth noting that under this system there is no place on the system to go to get the name of the file you are using (/etc/localtime may not be a link, and may just be a copy, or its own database not represented in /usr/share/zoneinfo). Additionally, the names of the files in the system zoneinfo database follow an internal standard, and there is no established standard for naming timezones. So even if you were using one of these files, and you did know its name, you cannot assume that that name matches any timezone specified by any other system or description.

One common misconception about time zones is that the standard time zone abbreviations can be used. For instance, EST surely refers to Eastern Standard Time. This is unfortunately not true - CST can refer to China Central Time, Central Standard Time, or Cuba Summer Time for instance - and time zone libraries that appear to correctly parse times that use time zone abbreviations do so by using a heuristic that usually assumes you mean a time in the US or Europe, in that order. Time zones also sometimes use two different abbreviations depending on whether the time in question is in standard time, or daylight savings time. These abbreviations are kept in the timezone databases, which is how programs like date manage to output meaningful abbreviations, it is only reading in times with abbreviations that is poorly specified.

This library contains a function that attempts to make an accurate determination of the machine timezone by testing the md5 sum of the currently referenced timezone file against all of the possible candidates in the system database. It additionally makes some adjustments to return the more common timezone names since some files in the database are duplicated under several names. It returns an option because of the problems mentioned above.

## The problems with string time conversions

There are two cases where string time conversions are problematic, both related to daylight savings time.

In the case where time jumps forward one hour, there are possible representations of times that never happened 2006-04-02T02:30:00 in the eastern U.S. never happened for instance, because the clock jumped forward one hour directly from 2 to 3. Unix time zone libraries asked to convert one of these times will generally produce the epoch time that represents the time 1/2 hour after 2 am, which when converted back to a string representation will be T03:30:00.

The second case is when the clocks are set back one hour, which causes one hour of time to happen twice. Converting a string in this range without further specification into an epoch time is indeterminate since it could be referring to either of two times. Unix libraries handle this by either allowing you to pass in a dst flag to the conversion function to specify which time you mean, or by using a heuristic to guess which time you meant.

The existence of both cases make a strong argument for serializing all times in UTC, which doesn't suffer from these issues.