Introducing Python, 2nd Edition

Chapter 13. Calendars and Clocks

“One!” strikes the clock in the belfry tower, Which but sixty minutes ago Sounded twelve for the midnight hour.

Frederick B. Needham, The Round of the Clock

I’ve been on a calendar but I have never been on time.

Marilyn Monroe

Programmers devote a surprising amount of effort to dates and times. Let’s talk about some of the problems they encounter and then get to some best practices and tricks to make the situation a little less messy.

Dates can be represented in many ways—too many ways, actually. Even in English with the Roman calendar, you’ll see many variants of a simple date:

• July 21 1987

• 21 Jul 1987

• 21/7/1987

• 7/21/1987

Among other problems, date representations can be ambiguous. In the previous examples, it’s easy to determine that 7 stands for the month and 21 is the day of the month, because months don’t go to 21. But how about 1/6/2012? Is that referring to January 6 or June 1?

The month name varies by language within the Roman calendar. Even the year and month can have a different definition in other cultures.

Times have their own sources of grief, especially because of time zones and daylight savings time. If you look at a time zone map, the zones follow political and historic boundaries rather than crisp lines every 15 degrees (360 degrees / 24) of longitude. And countries start and end daylight savings times on different days of the year. Southern hemisphere countries advance their clocks as their northern friends are winding ...