Time Usage With Perl

Brief summary of tips for using time functions in Perl:

• GMT = Grenwich Mean Time = UTC = Universal Coordinated Time. UTC is now the preferred usage.

   

• If your programs are returning correct time results and continue to return correct time results through time zone changes, then you are using the functions correctly.

   

• If you constantly have to modify your programs (adding an hour, subtracting an hour) whenever time zones change, then you are not using the functions correctly. (Keep reading )

   

• Time functions and what they take as arguments and what they return:

But first a definition:

Some functions take a seconds argument and return the various date/time elements of seconds, hours, month, etc. I refer to this as the broken-down time list.

A broken-down time list example:
my ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst);

Functions returning a list of broken-down time elements will return them in the order listed above.

0 <= sec <= 60

0 <= min <= 59

0 <= hour <= 23

1 <= mday <= 31, the day of the month

0 <= mon <= 11 , 0=Jan, 1=Feb, 2=Mar, etc.

year has 1900 subtracted from it, i.e. years since 1900. The year 1999 is 99 and the year 2000 is 100.

0 <= wday <= 6 , the day of the week , 0=Sunday, 1=Monday, 2=Tuesday, etc.

1 <= yday <= 366 , the day of the year

isdst = {0,1} , 0=DST (Daylight Saving Time) not in effect, 1= DST in effect

The isdst argument will rarely be used and is not needed to accurately display the time, so we will ignore it in the rest of our discussion.

Now, the functions:

(times)[0] - returns CPU seconds and fractions of a second that the current process has been executing.

time() - returns the number of non-leap seconds since the epoch (January 1, 1970 UTC, on Unix systems)

gmtime(seconds) - returns list of broken-down time representing seconds in UTC
ex: ($sec,$min,$hour,etc...) = gmtime(time)
returns the current time in UTC. (isdst is always equal to 0 for UTC.)

timegm(broken-down time) - returns seconds
ex: $gmTimeInSeconds = timegm($sec,$min,$hour, etc...)

localtime(seconds) - returns list of broken-down times
ex: ($sec,$min,$hour,etc...) = localtime(time)
returns the current local time based on the current time zone setting (see #9, below)

timelocal(broken-down time) - returns seconds
ex: $localTimeInSeconds = timelocal($sec,$min,$hour,etc...)

• The timegm() and timelocal() functions will need a use statement:
  use Time::Local;

   

• If you only care about timing something within the current process (script), use (times)[0] called at the
  appropriate places. However, if you need to attach date/time significance to things, use the other functions.

   

• If you are timing how long something takes then you can just perform all calculations using gmtime() and timegm(), assuming you need the time/date functionality and the Perl times() is not sufficient.

   

• If you need to display a date/time in anything other than UTC, then convert the time at the point that you need to display it using localime(time in seconds).

   

• Time Zones: localtime() and timelocal() will always represent the time based on the value of the time zone environment variable if it has been set, or the value of the server's "time zone symbolic link" if the time zone environment variable has not been set.

The FutureQuest^® server's time zone symbolic link is stored at /etc/localtime. This symbolic link currently points to /usr/share/zoneinfo/US/Eastern, the time zone in which the server is located.

The time zone environment variable is TZ. You set it like this:
$ENV{TZ} = ':/usr/share/zoneinfo/US/Eastern';

This sets the TZ environment variable to the US/Eastern time zone. Note the use of the leading ":" which is required. You can use 'ls' from telnet to get a listing of all the time zones contained in the /usr/share/zoneinfo directory and its subdirectories. Or, you can visit www.timezoneconverter.com/cgi-bin/tzref.tzc which lists all the timezones that can be used. Simply tack on whatever time zone you want to use to the string ':/usr/share/zoneinfo/'. Note that all time zone usage is case sensitive.

Example #1:

You have performed all your time/date operations using gmtime() and timegm() and now have your date/time represented as SECONDS stored in $TimeInSeconds and you want to display this time in the Europe/Paris time zone.

use Time::Local;
my @dayofweek = (qw(Sunday Monday Tuesday Wednesday Thursday Friday Saturday));
my @monthnames = (qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec));
my ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday);
$ENV{TZ} = ':/usr/share/zoneinfo/Europe/Paris';
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) = localtime($TimeInSeconds);
$year += 1900;
print "This date is $dayofweek[$wday], $monthnames[$mon] $mday, $year\n";
print "This time is $hour:$min:$sec\n";

Example #2:

You have performed all your time/date operations using gmtime() and timegm() and now have your date/time represented as a UTC broken-down time stored in the variables $sec, $min, etc., and you want to display this time in the Europe/Paris time zone.

use Time::Local;
my $TimeInSeconds;
#convert from UTC to Europe/Paris
$ENV{TZ} = ':/usr/share/zoneinfo/Europe/Paris';
#convert broken-down time to seconds in UTC
$TimeInSeconds = timegm($sec,$min,$hour,$mday,$mon,$year,$wday,$yday);
#convert the UTC seconds to broken-down time in time zone
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) = localtime($TimeInSeconds);

• When you display a time, ALWAYS, ALWAYS, ALWAYS display the time zone that the time represents so your reader can give the time some context.

   

• When you store a date/time variable (such as in a file), ALWAYS, ALWAYS, ALWAYS store the variable in UTC.

The above was submitted by Rich from TimeZoneConverter.com

For clean code, it is recommended to use File::stat module which is part of the standard distribution in version 5.004 and later. Use method mtime() to get the last modification time. This is the time the file itself was modified last. Instead you can also use stat.ctime which is the time directory information about the file was changed, not the file itself. stat.atime is the last time the file was accessed.         use File::stat; use Time::localtime; $datetime_string = ctime(stat($file)->mtime); print "file $file was updated at $datetime_string\n";

Newbee21369 has asked for the wisdom of the Perl Monks concerning the following question:
I'm trying to get the timestamp of a file in a directory. When I run the code shown below I get the following result.

Last change: Time::tm=ARRAY(0x200b6064)

How can I get the this format as my Result?
Last change: 2004111622

#!/usr/bin/perl

use Time::localtime;
$tm = localtime;

opendir(DIR,"/usr/path");
my @dir=grep { !/^\.+$/ } readdir(DIR);
closedir(DIR);
$file_count = 0;

foreach $file (@dir)
{
$mtime = (stat ($file))[9];
print "Last change:\t" . scalar localtime($mtime) . "\n";
}

2004-11-26 Janitored by Arunbear - added code tags, as per Monastery guidelines

Comment on Get file timestamp for a file in a directory
Select or Download Code
Re: Get file timestamp for a file in a directory
by steves (Curate) on Nov 26, 2004 at 19:03 UTC
Surround your code with <code> </code> tags so we can read it.
[reply]
Re: Get file timestamp for a file in a directory
by ikegami (Sage) on Nov 26, 2004 at 19:07 UTC
Change
print "Last change:\t" . scalar localtime($mtime) . "\n";
to
print "Last change:\t" . ctime(localtime($mtime)) . "\n";
as mention in the Time::localtime docs.
[reply]
[d/l]
[select]
Re^2: Get file timestamp for a file in a directory
by ikegami (Sage) on Nov 26, 2004 at 21:01 UTC
Now that your post is readable, I'd like to correct myself. POSIX::strftime will suit your formating needs, and you need to add the path to the file you're stating for it to work properly. Here's working code:

#!/usr/bin/perl

use strict;
use warnings;

use POSIX ();

my $dir = "/usr/path";

opendir(DIR, $dir);
my @dir = grep { !/^\.+$/ } readdir(DIR);
closedir(DIR);

foreach (@dir)
{
my $file = "$dir/$_";
my $mtime = (stat($file))[9];
print "Last change:\t"
. POSIX::strftime("%Y%m%d", localtime($mtime))
. "\n";
}

[reply]
[d/l]
[select]
Re: Get file timestamp for a file in a directory
by steves (Curate) on Nov 26, 2004 at 19:28 UTC
Yes, your use of Time::localtime is the key. It overrides the built-in localtime. If you use the built-in, it returns a ctime(3) like string. But in either case, a ctime string isn't going to give you that 2004111622 format in most cases. To get a specific date format, you'd also want to incorporate something like a call to the POSIX strftime function.

I can't tell for sure, but it looks like you may also be trying to stat the base file name only: since you open the directory, you'd need to prepend the directory. Otherwise it will only work if you run it from that directory.
[reply]
Re: Get file timestamp for a file in a directory
by Newbee21369 (Novice) on Nov 27, 2004 at 16:45 UTC
Thanks for all your help!

 

Etc

Society

Groupthink : Two Party System as Polyarchy : Corruption of Regulators : Bureaucracies : Understanding Micromanagers and Control Freaks : Toxic Managers :   Harvard Mafia : Diplomatic Communication : Surviving a Bad Performance Review : Insufficient Retirement Funds as Immanent Problem of Neoliberal Regime : PseudoScience : Who Rules America : Neoliberalism  : The Iron Law of Oligarchy : Libertarian Philosophy

Quotes

War and Peace : Skeptical Finance : John Kenneth Galbraith :Talleyrand : Oscar Wilde : Otto Von Bismarck : Keynes : George Carlin : Skeptics : Propaganda  : SE quotes : Language Design and Programming Quotes : Random IT-related quotes :  Somerset Maugham : Marcus Aurelius : Kurt Vonnegut : Eric Hoffer : Winston Churchill : Napoleon Bonaparte : Ambrose Bierce :  Bernard Shaw : Mark Twain Quotes

Bulletin:

Vol 25, No.12 (December, 2013) Rational Fools vs. Efficient Crooks The efficient markets hypothesis : Political Skeptic Bulletin, 2013 : Unemployment Bulletin, 2010 :  Vol 23, No.10 (October, 2011) An observation about corporate security departments : Slightly Skeptical Euromaydan Chronicles, June 2014 : Greenspan legacy bulletin, 2008 : Vol 25, No.10 (October, 2013) Cryptolocker Trojan (Win32/Crilock.A) : Vol 25, No.08 (August, 2013) Cloud providers as intelligence collection hubs : Financial Humor Bulletin, 2010 : Inequality Bulletin, 2009 : Financial Humor Bulletin, 2008 : Copyleft Problems Bulletin, 2004 : Financial Humor Bulletin, 2011 : Energy Bulletin, 2010 : Malware Protection Bulletin, 2010 : Vol 26, No.1 (January, 2013) Object-Oriented Cult : Political Skeptic Bulletin, 2011 : Vol 23, No.11 (November, 2011) Softpanorama classification of sysadmin horror stories : Vol 25, No.05 (May, 2013) Corporate bullshit as a communication method  : Vol 25, No.06 (June, 2013) A Note on the Relationship of Brooks Law and Conway Law

History:

Fifty glorious years (1950-2000): the triumph of the US computer engineering : Donald Knuth : TAoCP and its Influence of Computer Science : Richard Stallman : Linus Torvalds  : Larry Wall  : John K. Ousterhout : CTSS : Multix OS Unix History : Unix shell history : VI editor : History of pipes concept : Solaris : MS DOS :  Programming Languages History : PL/1 : Simula 67 : C : History of GCC development :  Scripting Languages : Perl history   : OS History : Mail : DNS : SSH : CPU Instruction Sets : SPARC systems 1987-2006 : Norton Commander : Norton Utilities : Norton Ghost : Frontpage history : Malware Defense History : GNU Screen : OSS early history

Classic books:

The Peter Principle : Parkinson Law : 1984 : The Mythical Man-Month :  How to Solve It by George Polya : The Art of Computer Programming : The Elements of Programming Style : The Unix Hater’s Handbook : The Jargon file : The True Believer : Programming Pearls : The Good Soldier Svejk : The Power Elite

Most popular humor pages:

Manifest of the Softpanorama IT Slacker Society : Ten Commandments of the IT Slackers Society : Computer Humor Collection : BSD Logo Story : The Cuckoo's Egg : IT Slang : C++ Humor : ARE YOU A BBS ADDICT? : The Perl Purity Test : Object oriented programmers of all nations : Financial Humor : Financial Humor Bulletin, 2008 : Financial Humor Bulletin, 2010 : The Most Comprehensive Collection of Editor-related Humor : Programming Language Humor : Goldman Sachs related humor : Greenspan humor : C Humor : Scripting Humor : Real Programmers Humor : Web Humor : GPL-related Humor : OFM Humor : Politically Incorrect Humor : IDS Humor : "Linux Sucks" Humor : Russian Musical Humor : Best Russian Programmer Humor : Microsoft plans to buy Catholic Church : Richard Stallman Related Humor : Admin Humor : Perl-related Humor : Linus Torvalds Related humor : PseudoScience Related Humor : Networking Humor : Shell Humor : Financial Humor Bulletin, 2011 : Financial Humor Bulletin, 2012 : Financial Humor Bulletin, 2013 : Java Humor : Software Engineering Humor : Sun Solaris Related Humor : Education Humor : IBM Humor : Assembler-related Humor : VIM Humor : Computer Viruses Humor : Bright tomorrow is rescheduled to a day after tomorrow : Classic Computer Humor

The Last but not Least Technology is dominated by two types of people: those who understand what they do not manage and those who manage what they do not understand ~Archibald Putt. Ph.D

The statements, views and opinions presented on this web page are those of the author (or referenced source) and are not endorsed by, nor do they necessarily reflect, the opinions of the Softpanorama society. We do not warrant the correctness of the information provided or its fitness for any purpose. The site uses AdSense so you need to be aware of Google privacy policy. You you do not want to be tracked by Google please disable Javascript for this site. This site is perfectly usable without Javascript.

Last modified: March, 12, 2019