## Monday, 3 January 2011

When did the current decade begin? Last Saturday? Or 1 January 2010? A decade is a period of ten years. But when does a decade begin?

It's an interesting mathematical phenomenon that the number systems we use for the calendar do not start with zero. We number the days of the months from 1 to 31. We number the months of the year from 01 to 12. And we number the years (BC) from 1 BC onwards. The year before 1 BC we call 1 AD, missing out zero! We don't have a day zero each month, or a month zero each year, or a year zero BC. (But note that, inconsistently, we do have a time of zero hours, zero minutes and zero seconds in the 24-hour clock system!)

On this basis, the first decade would have been years 1–10 BC. And the current decade would be 2011–2020, beginning at midnight on 01/01/2011.

However ... it is convenient to refer to decades as, for example, 'the roaring forties' (1940–1949) or, recently, the 'noughties' (2000–2009), in which case the decade begins with a year ending with a zero. On that basis, the current decade would be 2010–2019, and would have begun on 01/01/2010.

A mathematical argument would favour a decade beginning on the 1st January in a year ending with the digit '1'. But a linguistic argument would favour the year ending with a zero. Which is better? Not for me to say, of course.

But, I wish you a happy new decade all the same.