Gregorian Calendar

The Gregorian calendar is the most widely used civil calendar in the world.
A year starts on January, 1st and ends on December, 31st.

👉 This allows you to operate quick comparison between months, however those have different lenghts.

ISO Week Date Calendar

An ISO week-numbering year (also called ISO year informally) has 52 or 53 full weeks. That is 364 or 371 days instead of the usual 365 or 366 days. The extra week is sometimes referred to as a leap week.

In ISO Year, 

  • the whole year is divided in 4 periods. 
  • Each period groups 13 weeks. 
  • And each week counts 7 days.
  • Weeks start with Monday.

👉 This allows you to compare periods of time having the same number of days.

Comparison is more accurate than Gregorian Calendar, as it prevents from comparing February having 28 days against March having 31 days. 

3 business days can make a huge difference !

The first week of the year starts on 

The first week of the year, hence, always contains 4 January. ISO week year numbering therefore slightly deviates from the Gregorian for some days close to 1 January.

So keep an eye on the period you're acually looking at !

In 2019, the first day of the first week was the Monday 31 of December.

Did this answer your question?