# ISO week date  ## About - Author: wikipedia.org - Title: ISO week date - Tags: #articles - URL: https://en.wikipedia.org/wiki/ISO_week_date ## Highlights Weeks start with Monday and end on Sunday. Each week's year is the Gregorian year in which the Thursday falls. The first week of the year, hence, always [contains 4 January](https://en.wikipedia.org/wiki/ISO_week_date#First_week). ISO week [year numbering](https://en.wikipedia.org/wiki/Year_numbering) therefore usually deviates by 1 from the Gregorian for some days close to 1 January. - Note: I believe that the obsidian periodic notes use locale custom week of year and not ISO, meaning some dates may be off in the calendar display. --- The ISO week year number deviates from the Gregorian year number in one of three ways. The days differing are a Friday through Sunday, or a Saturday and Sunday, or just a Sunday, at the start of the Gregorian year (which are at the end of the previous ISO year) and a Monday through Wednesday, or a Monday and Tuesday, or just a Monday, at the end of the Gregorian year (which are in week 01 of the next ISO year). In the period 4 January to 28 December the ISO week year number is always equal to the Gregorian year number. The same is true for every Thursday. - Note: Thus doing finances on a thursday means that the dates will always align with the gregorian calendar I.e. Thursday will always correspond to the same year in BOTH ISO and Gregorian - removing confusion about what year a payment was made etc. --- Thus, within a 400-year cycle: • 27 week years are *5 days longer* than the month years (371 − 366). • 44 week years are *6 days longer* than the month years (371 − 365). • 70 week years are *2 days shorter* than the month years (364 − 366). • 259 week years are *1 day shorter* than the month years (364 − 365). - Note: So at most a week year financial system may be off from a month year financial system by 6 days. ---