Question 1

What is an ISO week number?

Accepted Answer

ISO 8601 defines a week numbering system where weeks run Monday through Sunday. Week 1 of a year is the week containing the first Thursday of that year (equivalently, the week containing January 4th). Years can have either 52 or 53 ISO weeks. The ISO week year may differ from the calendar year at the boundaries.

Question 2

Why can a date in January have the previous year's ISO week number?

Accepted Answer

If January 1 falls on a Friday, Saturday, or Sunday, the week containing it has more days in December than January, so that week belongs to the previous year. For example, January 2, 2021 falls in ISO week 2020-W53. Conversely, some late-December dates belong to week 1 of the following year.

Question 3

What is the difference between week number and day of year?

Accepted Answer

The day of year (ordinal date) is a simple count from January 1 = day 1 through December 31 = day 365 (or 366 in a leap year), with no regard for weekday. The ISO week number groups consecutive 7-day periods (Monday-Sunday) into numbered weeks, which may span across year boundaries.

Question 4

What is a fiscal quarter and how does Q1 Q2 Q3 Q4 correspond to months?

Accepted Answer

A fiscal quarter divides the year into four 3-month periods. The standard Gregorian quarters are: Q1 = January–March, Q2 = April–June, Q3 = July–September, Q4 = October–December. Some organizations use fiscal years that start in a different month, but this tool uses the standard calendar year definition.

Question 5

How does the week-to-date conversion work?

Accepted Answer

Given an ISO year and week number, the tool finds the Monday of that week. It locates January 4th (which is always in ISO week 1), finds that week's Monday, then adds (weekNumber - 1) × 7 days to get the Monday of the target week. Sunday is Monday + 6 days.

Calendar Week Calculator

Date to Week Number

Week to Date Range

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