You should never use it in a day-month-year situation, but it is very useful in a weekofyear-year situation commonly encountered in bookkeeping. You have to de

You should never use it in a day-month-year situation, but it is very useful in a weekofyear-year situation commonly encountered in bookkeeping. You have to decide what to do with the week that gets broken up when a new year starts, and the ISO standard makes sure everybody uses the same week numbers. I personally experienced how this mattered when someone in our company was unaware of ISO and decided to invent an other week schema as new standard. First came the people who complained outlook numbered them differently. Then the complaints that all other companies delivered using another system. Then it turns out data transfers from the government didn't match either. Finally one guy pops up and sends the link to ISO in wikipedia. Our whole 1000 man part of the org was in CC of the original mail, and naturally we received all reply complaints too. After a very noisy few days in the inbox, all of us knew all about ISO weeks and why they matter. Training couldn't be better.