And for many people on a biweek schedule, this year is a 27 pay period year, so there are even more extra pay checks. I think those happen every 11 years or so.

For anyone to whom this year is still a 26 pay period year, next year is your 27 pay period year. Unfortunately, I know from what I've seen my own employer do with salaries employees is that they'll just divide our salary by 27 next year so each paycheck is smaller next year, but we'll get 27 of them. If you're salaried, that's actually the right way to do it since the employer's agreement is a set number of dollars paid to you per calendar year.

Your pay is based per year, but you are paid bi-weekly (salary/365*14). You are assuming all paychecks in a calendar year are for work within the calendar year, which is obviously not true. That first paycheck is for work in the previous year.

Let's apply some math to this problem.

Assume the following:

- Your contractual agreement is that you are paid $100,000 per year.
- You are paid biweekly so you are paid $3846.15 every two weeks.
- You work at the same company for 25 years and then immediately quit after the 25 years are over.
- The employer does not adjust paycheck amounts for 27 paycheck years.

How much money will you have received in total after the 25 years of employment?

First we need to determine how many weeks are in 25 years. This can vary, but for our purposes we will assume that a year has 365.242 days, so 25 years has 9131.05 days or 1304.44 weeks, which means 652.22 paychecks. So your total pay received after 25 years is approximately $2,508,536.

However, according to your pay agreement you were only "supposed" to receive $2,500,000, so you did indeed get some extra money over the course of 25 years, namely $8,536 in extra money after 25 years.

Now, the next question is, does adjusting paycheck amounts for a 27 paycheck year result in the "right" amount of money being paid, or does it cheat you out of money?

For that, we need to find how many 27 paycheck years occur in 25 years.

We said there were 1304.44 weeks in 25 years, or 52.1776 weeks per year. This also means there are 52.1776 Fridays per year. Thus, a year contains on average 26.0888 paydays. Thus, there will be a 27 paycheck year every 1/0.0888=11.26 years. Thus, 25 years contains on average 2.22 years with 27 paychecks.

The next question is: how much is your pay reduced by in a year if the employer adjusts paychecks for a 27 paycheck year? The answer is that your pay is reduced by 3846.15*27-100000 = $3,846.05. So over the course of 25 years, you would lose 2.22 times of that, namely $8,538.23, compared to an employer who does not modify paychecks for 27 paycheck years.

We already found that after 25 years, an employee whose paychecks are not modified for 27 paycheck years would have received $2,508,536. We now subtract the $8,538 we found above and we arrive at $2,500,000, the exact amount you were entitled to be paid under the employment contract.

In conclusion, an employer who does not modify paychecks for a 27 paycheck year is giving a small bonus to employees, and is not required to do so under the employment contract (assuming the contract is that you are paid $100,000 per year). Modification of the paycheck amount for 27 paycheck years is mathematically sound assuming you work long enough.

There is, however, a major caveat. Depending on when you leave, you might actually be slightly underpaid such that the employer would have to pay you a "true up". If your employer does not have a process for calculating this, they may be engaging in illegal wage theft. For example, if you leave after the first day of a 27 paycheck year, you would be owed a true up because that upcoming paycheck is actually for work in the previous year. If you stay employed long enough, this averages out, but if you quit at the "wrong" time, you are cheated out of wages. The modification only works out in the long term. In the short term, you may be underpaid at times. Thus, an employer needs to be prepared to calculate "true up" amounts for departing employees.

However, if your contract is that you are paid a certain amount per two weeks, none of this applies, and an employer obviously could not adjust the pay for 27 paycheck years.