We’ve been debating this internally for about four months now: Everyone knows that there are 52 weeks in a year, right? Unfortunately, it’s never exactly 52, since 365 (or 366 in leap years) are not evenly divisible by seven.

Project managers regularly track their teams’ work by numbered weeks, so it’s not surprising that project reports need to be able to show which activities appear in a given week with some consistency. Believe it or not, there is not a consensus as to which week is which, or even when one week ends and the next begins.

The examples below show three common ways to count week numbers. January 1st, 2016 is highlighted in red as a point of reference. You can click on each image to enlarge.

ISO 8601: January 4th

Most of Europe, Asia, and Oceania adhere to the ISO 8601 standard, which dictates that weeks start on a Monday, and that the first week of the new year is the week that contains January 4th. With ISO 8601, week #53 of 2015 bleeds over three days into 2016:

January 1st

In the Americas, Japan, Israel, and South Africa, weeks start on Sunday instead of Monday. Many companies in these regions define the first week of the year as the week that contains January 1st. With this in mind, there were only 52 weeks in 2015, but week #1 of 2016 extends five days backwards into 2015:

First Full Week

Not everyone who starts their weeks on a Sunday treats January 1st as the first week. Other companies–many of whom are next-door neighbors to the ones above–define the first week of the year as the first full week of the year. In 2015, this meant that January 1st, 2016 was part of week #52 of 2015:

Is there a right or wrong answer here? Probably not. However, one thing is certain: If you share your schedules externally and rely on week numbers to track deliverables, make sure that all of the stakeholders in your project define weeks in the same manner. Otherwise, sticking to good old months and days might be a good idea.