The formula
length of service = complete years, months and days between joining and the cut-off date
Where it comes from
Length of service is counted the way people talk about it, moving month by month from the joining date, not by dividing days by 365. Dividing gives different answers depending on how many leap years fall inside the period, and it does not match what an employment agreement says.
How to work it out by hand
- Start from the date you joined the company
- Count the complete years up to the cut-off date
- Carry on with the complete months left over
- Whatever days remain are the remainder
What is worth knowing
The gap between counting in months and dividing by 365 is not theoretical. Four years and eleven months is four completed years, not five, and long-service payments are often made on completed three- or five-year blocks: reaching the day after the anniversary changes the amount. That is why the calculator also gives the exact date of the next anniversary and the days still to go.