Perl Cookbook, 2nd Edition

Chapter 3. Dates and Times

It is inappropriate to require that a time represented as seconds since the Epoch precisely represent the number of seconds between the referenced time and the Epoch.

—IEEE Std 1003.1b-1993 (POSIX) Section B.2.2.2

Introduction

Times and dates are important things to be able to manipulate. “How many users logged in last month?”, “How many seconds should I sleep if I want to wake up at midday?”, and “Has this user’s password expired yet?” are common questions whose answers involve surprisingly non-obvious manipulations.

Perl represents points in time as intervals, measuring seconds past a point in time called the Epoch. On Unix and many other systems, the Epoch was 00:00 Jan 1, 1970, UTC (Universal Corrected Time).^[1]

When we talk about dates and times, we often interchange two different concepts: points in time (dates and times) and intervals between points in time (weeks, months, days, etc.). Epoch seconds represent intervals and points in the same units, so you can do basic arithmetic on them.

However, people are not used to working with Epoch seconds. We are more used to dealing with individual year, month, day, hour, minute, and second values. Furthermore, the month can be represented by its full name or its abbreviation. The day can precede or follow the month. Because of the difficulty of performing calculations with a variety of formats, we typically convert human-supplied strings or lists to Epoch seconds, calculate, and then convert back to strings ...