Tonight expect a lengthier evening, as an extra leap second is going to be added to machines and devices which mark, or tell time. It's called a leap second.
The International Earth Rotation and Reference Systems Service coordinates these leap seconds, and such coordination typically takes place in either June or December. This weekend's skip: Universal time will be 11:59:59 and then the rarely seen 11:59:60 before the clocks strike midnight. A longer day indeed.
Historically, we humans have based time on the average rotation of the Earth relative to other celestial bodies, with the second defined by this frame of reference. The next leap second will be added at 23:59:60 tonight, June 30, 2012, or is it technically tomorrow morning?