This is more of a quick alert since many business people are not aware that for many employers this will be a 'leap' pay period year. What that means is that many employers who pay their employees bi-weekly will have 27 pay periods in 2009 rather than the normal 26 pay periods. This has been written about a few other places including here.
Leap pay years happen just like regular leap years because there is an extra day in a year that is not accounted for by 26 pay periods of 14 days each (26X14=364). Eventually that extra time has to be accounted for which results in an extra pay period.
For many employers that will be in 2009. This will mainly impact employers who had their first pay day of 2009 the very first week of January.
One way that this will creep up and surprise many employers is that 1/1/2010 is a Friday. Most companies elect to pay their employees a day earlier if the pay date falls on a holiday. Companies that follow this protocol will have their final pay date of 2009 hitting on Thursday 12/31/2009 rather than Friday 1/1/2010.
Companies that account their payroll on an accrual basis will still just have their normal payroll expense. Companies that do not accure their expenses will have an additional expense in 2009.
Big areas that companies do need to montior include employee deductions and matching contributions. If a company is not diligent it is possible that employees may be overwitheld or overpaid for these areas.
The more likely impact of this is on employees though. Since virtually every individual is on a cash basis for tax purposes. This will mean that they will receive more income on their 2009 W2s. There are all kind of impacts to this including potentially being in a higher tax bracket and having to double check with their employer that they do not exceed the limits on retirement plan contributions, child support payments and the like.
It's this kind of stuff that makes the world of payroll and HR so much fun.