Module Core__.Core_time_intf

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.