Derick Rethans one of the go-to guys for working with time in PHP has made this new post about something 2008 picked up along the way to 2009 - a leap second.
The start of this new year started with some buzz about a leap second being introduced between Dec 31st 2008, 23:59:59 and Jan 1st 2009, 00:00:00. I've had people ask where this leap second actually comes from, and whether you need to worry about it in your applications. To understand leap seconds means, unfortunately, understanding how time is actually kept.
He ponints out one of the major problems - how time is kept. With variants of Universal Time, it makes it hard to track down what's "right". He breaks out the difference between other time storage methods and the unix time that PHP can use (that counts the number of seconds since Jan 1st 1970) and how the leap second was handled for each.