How do libraries in different programming languages handle Date & Time, Timestamps & Durations, Leapseconds & -years, DSTs & Timezones, …?

Question

Is there a standard body or a specific normative way how time-related things should be implemented in practice (like ICU for Unicode-related tasks) or is this curre

Answer 1

There are time(s) and there are dates (calendars)
The first problem is that dates are not linked to time but to astronomical position of Earh, Moon, etc. + regularity/periodicity of human activity. The time is also subjective and relative or even relativistic and measured either astronomically or or atomically.

The time bodies and the date/calendar bodies
The International Standartization Organization (ISO) [4] has issued

• "ISO 8601 Data elements and interchange formats — Information interchange — Representation of dates and times" [4a]

which, like other international standards, is recommendation and is based on already established practices.
It is (subjectively) based only on Gregorian Calendar [5] and on proleptic one (projected backwards to well before it was actually invented so is of limited use in dealing with historic dates) [5a].

The World Calendar Association [1d] initiated the introduction of new World Calendar since 2012 [1b-1d] which would make useless already existing date libraries. Again, the same main problem, see on it further on.

The most covering, I ever saw, date-time-related comparison in IT systems is [2] between BIG8 DBMS (IBM DB2, Informix, Ingres, InterBase, Microsoft SQL Server, MySQL, Oracle, and Sybase).
This and all other surveys show that the processing of the same, for ex., Gregorian calendar time/dates are different between all systems as well as inside the same platforms (between different products and versions of the same product), see for ex., [3].

THE MAIN PROBLEM with all date/time libraries in all-all systems, frameworks is that their date/time datatypes do not permit to include geographical and calendar information in date/time datatypes.
Without which they are mainly half- useless - what is the sense of milliseconds in SQL Server datetime2 values in 7th century? At that time there was even no clocks measuring time with accuracy of minutes (Galileo Galilei used, for ex., his heart beats to measure intervals of time in his experiments) as well as Gregorian Calendar was not even invented.

So, plenty of datetime type space is misused failing to give the most important flexibility of working with dates by linking and including with them to geography and/or calendar info.

Just fast illustrations:

• Modern Russia uses Gregorian calendar and the Russian Orthodox Church uses Julian calendar by which many state holidays in Russia are determined (for example, Christmas in Russia is on 7th of January and Old New Year is on 14th of January by Gregorian Calendar while the dates of other religious holidays are floating relatively to Gregorian Calendar).
• In pre-1917 Russia, Poland as its part, used Gregorian Calendar while all the rest Russia used Julian Calendar (with floating difference of 13-18 days at the "same" time zone) [5b];
• Double-click on clock in MS Windows (or open Control Panel --> Date and Time) --> Time Zone tab --> view time zones in combobox. You will see that there are 25 hours from GMT-12:00 to GMT+13:00 over a hundred of time zones with fractions of hour like GMT+5:00, GMT+5:30, GMT+5:45, etc.

==== Cited:
[1] New World Calendar
[1a]Update: Sorry, do not read [1a], author confused calendars and wrote wrong info in this news brief
World Calendar 2012: 35 days in a month
http://www.panorama.am/en/society/2010/01/29/newcalendar
[1b] http://en.wikipedia.org/wiki/World_Calendar
[1c] http://www.theworldcalendarin2012.org/Index2.htm
[1d] http://www.theworldcalendar.org/TWCA.htm

[2] Peter Gulutzan,Trudy Pelzer. SQL Performance Tuning: Dates in SQL
http://www.informit.com/articles/printerfriendly.aspx?p=30939

[3] SqlDateTime.MinValue != C# DateTime.MinValue, why?
SqlDateTime.MinValue != DateTime.MinValue, why?

[4]
International Organization for Standardization
http://en.wikipedia.org/wiki/International_Organization_for_Standardization
[4a] ISO 8601 Data elements and interchange formats — Information interchange — Representation of dates and times
http://en.wikipedia.org/wiki/ISO_8601

[5]
Gregorian Calendar
http://en.wikipedia.org/wiki/Gregorian_calendar
[5a] Proleptic Gregorian calendar
http://en.wikipedia.org/wiki/Proleptic_Gregorian_calendar
[5b] Gregorian Calendar Adoption
http://en.wikipedia.org/wiki/Gregorian_calendar#Adoption
[6]
http://en.wikipedia.org/wiki/Galileo_Galilei