Re: [Unicon-group] Timezone

Thomas Christopher Fri, 18 Jan 2002 16:27:10 -0800

You might look at how the Python library handles times (including 
time zones). I've attached their html documentation page.
---
Thomas W. Christopher, [EMAIL PROTECTED]
http://tc.toolsofcomputing.com --- books (through Prentice Hall PTR): 
High-Performance Java Platform Computing, ISBN: 0130161640 
Web Programming in Python, ISBN: 0-13-041065-9 
Python Programming Patterns, ISBN: 0-13-040956-1




 
----- Original Message ----- 
From: "Clint Jeffery" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Cc: <[EMAIL PROTECTED]>
Sent: Friday, January 18, 2002 3:12 PM
Subject: Re: [Unicon-group] Timezone


> OK, let's pretend for a moment that we own the language and can add
> anything we want to it. :-)
> 
> What should our portable timezone information look like at the language
> level?  Should one of the existing keywords include timezone information?
> Should there be a new keyword?  Are there a standard set of strings
> worldwide for this?
> 
> I have not studied how to get the information under Windows, although I
> suspect it will be moderately easy, and that it might involve a call to
> GetTimeZoneInformation().
> 
> Clint
> 
> _______________________________________________
> Unicon-group mailing list
> [EMAIL PROTECTED]
> https://lists.sourceforge.net/lists/listinfo/unicon-group
>

Title: 6.9 time -- Time access and conversions

6.9 `time` -- Time access and conversions

This module provides various time-related functions. It is always available, but not all functions are available on all platforms.

An explanation of some terminology and conventions is in order.

The epoch is the point where the time starts. On January 1st of that year, at 0 hours, the ``time since the epoch'' is zero. For Unix, the epoch is 1970. To find out what the epoch is, look at gmtime(0).
The functions in this module do not handle dates and times before the epoch or far in the future. The cut-off point in the future is determined by the C library; for Unix, it is typically in 2038 .
Year 2000 (Y2K) issues: Python depends on the platform's C library, which generally doesn't have year 2000 issues, since all dates and times are represented internally as seconds since the epoch. Functions accepting a time tuple (see below) generally require a 4-digit year. For backward compatibility, 2-digit years are supported if the module variable accept2dyear is a non-zero integer; this variable is initialized to 1 unless the environment variable PYTHONY2K is set to a non-empty string, in which case it is initialized to 0. Thus, you can set PYTHONY2K to a non-empty string in the environment to require 4-digit years for all year input. When 2-digit years are accepted, they are converted according to the POSIX or X/Open standard: values 69-99 are mapped to 1969-1999, and values 0-68 are mapped to 2000-2068. Values 100-1899 are always illegal. Note that this is new as of Python 1.5.2(a2); earlier versions, up to Python 1.5.1 and 1.5.2a1, would add 1900 to year values below 1900.
UTC is Coordinated Universal Time (formerly known as Greenwich Mean Time, or GMT). The acronym UTC is not a mistake but a compromise between English and French.
DST is Daylight Saving Time, an adjustment of the timezone by (usually) one hour during part of the year. DST rules are magic (determined by local law) and can change from year to year. The C library has a table containing the local rules (often it is read from a system file for flexibility) and is the only source of True Wisdom in this respect.
The precision of the various real-time functions may be less than suggested by the units in which their value or argument is expressed. E.g. on most Unix systems, the clock ``ticks'' only 50 or 100 times a second, and on the Mac, times are only accurate to whole seconds.
On the other hand, the precision of time() and sleep() is better than their Unix equivalents: times are expressed as floating point numbers, time() returns the most accurate time available (using Unix gettimeofday() where available), and sleep() will accept a time with a nonzero fraction (Unix select() is used to implement this, where available).

The time tuple as returned by gmtime(), localtime(), and strptime(), and accepted by asctime(), mktime() and strftime(), is a tuple of 9 integers:

Index	Field	Values
`0`	year	(e.g. 1993)
`1`	month	range [1,12]
`2`	day	range [1,31]
`3`	hour	range [0,23]
`4`	minute	range [0,59]
`5`	second	range [0,61]; see (1) in `strftime()` description
`6`	weekday	range [0,6], Monday is 0
`7`	Julian day	range [1,366]
`8`	daylight savings flag	0, 1 or -1; see below

Note that unlike the C structure, the month value is a range of 1-12, not 0-11. A year value will be handled as described under ``Year 2000 (Y2K) issues'' above. A -1 argument as daylight savings flag, passed to mktime() will usually result in the correct daylight savings state to be filled in.

The module defines the following functions and data items:

accept2dyear: Boolean value indicating whether two-digit year values will be accepted. This is true by default, but will be set to false if the environment variable PYTHONY2K has been set to a non-empty string. It may also be modified at run time.

altzone: The offset of the local DST timezone, in seconds west of UTC, if one is defined. This is negative if the local DST timezone is east of UTC (as in Western Europe, including the UK). Only use this if daylight is nonzero.

asctime([tuple]): Convert a tuple representing a time as returned by gmtime() or localtime() to a 24-character string of the following form: 'Sun Jun 20 23:21:05 1993'. If tuple is not provided, the current time as returned by localtime() is used. Note: unlike the C function of the same name, there is no trailing newline.

clock(): Return the current CPU time as a floating point number expressed in seconds. The precision, and in fact the very definition of the meaning of ``CPU time'' , depends on that of the C function of the same name, but in any case, this is the function to use for benchmarking Python or timing algorithms.

ctime([secs]): Convert a time expressed in seconds since the epoch to a string representing local time. If secs is not provided, the current time as returned by time() is used. ctime(secs) is equivalent to asctime(localtime(secs)).

daylight: Nonzero if a DST timezone is defined.

gmtime([secs]): Convert a time expressed in seconds since the epoch to a time tuple in UTC in which the dst flag is always zero. If secs is not provided, the current time as returned by time() is used. Fractions of a second are ignored. See above for a description of the tuple lay-out.

localtime([secs]): Like gmtime() but converts to local time. The dst flag is set to 1 when DST applies to the given time.

mktime(tuple): This is the inverse function of localtime(). Its argument is the full 9-tuple (since the dst flag is needed; use -1 as the dst flag if it is unknown) which expresses the time in local time, not UTC. It returns a floating point number, for compatibility with time(). If the input value cannot be represented as a valid time, OverflowError is raised.

sleep(secs): Suspend execution for the given number of seconds. The argument may be a floating point number to indicate a more precise sleep time. The actual suspension time may be less than that requested because any caught signal will terminate the sleep() following execution of that signal's catching routine. Also, the suspension time may be longer than requested by an arbitrary amount because of the scheduling of other activity in the system.

strftime(format[, tuple])

Convert a tuple representing a time as returned by gmtime() or localtime() to a string as specified by the format argument. If tuple is not provided, the current time as returned by localtime() is used. format must be a string.

The following directives can be embedded in the format string. They are shown without the optional field width and precision specification, and are replaced by the indicated characters in the strftime() result:

Directive	Meaning	Notes
`%a`	Locale's abbreviated weekday name.
`%A`	Locale's full weekday name.
`%b`	Locale's abbreviated month name.
`%B`	Locale's full month name.
`%c`	Locale's appropriate date and time representation.
`%d`	Day of the month as a decimal number [01,31].
`%H`	Hour (24-hour clock) as a decimal number [00,23].
`%I`	Hour (12-hour clock) as a decimal number [01,12].
`%j`	Day of the year as a decimal number [001,366].
`%m`	Month as a decimal number [01,12].
`%M`	Minute as a decimal number [00,59].
`%p`	Locale's equivalent of either AM or PM.
`%S`	Second as a decimal number [00,61].	(1)
`%U`	Week number of the year (Sunday as the first day of the week) as a decimal number [00,53]. All days in a new year preceding the first Sunday are considered to be in week 0.
`%w`	Weekday as a decimal number [0(Sunday),6].
`%W`	Week number of the year (Monday as the first day of the week) as a decimal number [00,53]. All days in a new year preceding the first Sunday are considered to be in week 0.
`%x`	Locale's appropriate date representation.
`%X`	Locale's appropriate time representation.
`%y`	Year without century as a decimal number [00,99].
`%Y`	Year with century as a decimal number.
`%Z`	Time zone name (or by no characters if no time zone exists).
`%%`	A literal "`%`" character.

Notes:

(1): The range really is 0 to 61; this accounts for leap seconds and the (very rare) double leap seconds.

Here is an example, a format for dates compatible with that specified in the RFC 822 Internet email standard. ^6.1

>>> from time import *
>>> strftime("%a, %d %b %Y %H:%M:%S %Z", localtime())
'Sat, 27 Jan 2001 05:15:05 EST'
>>>

Additional directives may be supported on certain platforms, but only the ones listed here have a meaning standardized by ANSI C.

On some platforms, an optional field width and precision specification can immediately follow the initial "%" of a directive in the following order; this is also not portable. The field width is normally 2 except for %j where it is 3.

strptime(string[, format])

Parse a string representing a time according to a format. The return value is a tuple as returned by gmtime() or localtime(). The format parameter uses the same directives as those used by strftime(); it defaults to "%a %b %d %H:%M:%S %Y" which matches the formatting returned by ctime(). The same platform caveats apply; see the local Unix documentation for restrictions or additional supported directives. If string cannot be parsed according to format, ValueError is raised. Values which are not provided as part of the input string are filled in with default values; the specific values are platform-dependent as the XPG standard does not provide sufficient information to constrain the result.

Note: This function relies entirely on the underlying platform's C library for the date parsing, and some of these libraries are buggy. There's nothing to be done about this short of a new, portable implementation of strptime().

Availability: Most modern Unix systems.

time(): Return the time as a floating point number expressed in seconds since the epoch, in UTC. Note that even though the time is always returned as a floating point number, not all systems provide time with a better precision than 1 second.

timezone: The offset of the local (non-DST) timezone, in seconds west of UTC (i.e. negative in most of Western Europe, positive in the US, zero in the UK).

tzname: A tuple of two strings: the first is the name of the local non-DST timezone, the second is the name of the local DST timezone. If no DST timezone is defined, the second string should not be used.

See Also:

Module locale:: Internationalization services. The locale settings can affect the return values for some of the functions in the time module.

Footnotes

... standard.^6.1: The use of %Z is now deprecated, but the %z escape that expands to the preferred hour/minute offset is not supported by all ANSI C libraries. Also, a strict reading of the original 1982 RFC 822 standard calls for a two-digit year (%y rather than %Y), but practice moved to 4-digit years long before the year 2000.

See About this document... for information on suggesting changes.

Re: [Unicon-group] Timezone

6.9 time -- Time access and conversions

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6.9 `time` -- Time access and conversions