Perl for Web Site Management

Chapter 9. Date Arithmetic

This chapter continues the example begun in Chapter 8. In that chapter, we created a script to extract information from each line of our web server’s access log, and began work on the code to assemble “visit” statistics from that information. We had just reached the point where we wanted to use a subroutine called &get_seconds to convert from the date and time string included in our log file entries to something called the Epoch seconds. This &get_seconds subroutine is our first real exposure to the notion of date arithmetic . Let’s take a brief detour to look at that subject in more detail.

Date/Time Conversions

The idea with date arithmetic is that it’s much easier to do things like finding the interval between two date/time strings if you first convert those strings into a common format. The format of choice in this case is the Epoch seconds, which is the number of seconds since some specific point in time, called the Epoch. (On most Unix systems, the Epoch is 00:00:00 GMT/UTC on January 1, 1970.)

If you want to get the number of Epoch seconds corresponding to the current time, you can get it by using Perl’s time function, as in:

$seconds = time;

Two other Perl functions called localtime and gmtime can be used to convert Epoch seconds into human-readable local time or GMT (respectively). You may recall looking at localtime briefly in Chapter 6; now we’re going to look at it in more detail. Example 9-1 shows localtime and gmtime in action. (As usual, ...