Human Friendly Time

Perl possesses two built-in functions which happily translate the epoch time format into a more human friendly format. Both gmtime and localtime can be used to print out the current time and/or extract the various elements from an epoch time, such as month, hours, minutes, and so on. Whereas gmtime returns all of these values relative to Greenwich Mean Time, localtime adjusts its values to the local time on your own machine. This assumes, of course, that your operating system is properly configured to your time zone.

We'll look at the use of localtime, but remember that gmtime works exactly the same way and supports exactly the same syntax, but that it simply returns GMT time which is not corrected for your time zone.

The simplest way to use localtime is in "scalar context". For instance:

print scalar localtime(time);

Yields:

Wed Dec  8 10:16:31 1999

Remember that time represents the up-to-the-second epoch time; you can use localtime to print a human friendly version of any epoch time ... for example:

print scalar localtime(420221050);

Yields:

Tue Apr 26 12:04:10 1983

Right away, you can begin to see some potential usefulness in the way Perl views time in epoch seconds. Consider time calculations. Imagine that you know the current time, and you wish to calculate the date 24 hours ahead. Not as simple as it sounds if you think of the human calendar because some dates will rollover to a new month or even a new year when you add 24 hours. We needn't worry about these complications. Look at this way, instead: one day consists of 24 hours. An hour consists of 60 minutes, and a minute consists of 60 seconds. Therefore, a full day consists of 60*60*24, or 86,400 seconds.

$now=time;
$tomorrow=$now+86400;
print scalar localtime($tomorrow);

It is therefore quite simple to add or subtract periods of time, when you do all your calculations in epoch seconds. You then let Perl's localtime function worry about converting the epoch time into the proper year, day, month, and time. The handy table below summarizes a number of time periods and their epoch second equivalents.

Periods of time represented in seconds since the January 1, 1970 epoch.

1 hour	3600
1 day	86400
1 week	604800
2 weeks	1209600
3 weeks	1814400
1 month	2419200
6 months	14515200
1 year	29030400
1 decade	290304000

The Perl You Need to Know Part 9:The Millennium Episode -- Time and Date Manipulation
The Perl You Need to Know
Chopped Time