First let’s take a look at what ISO week is, from WikiPedia:

Week date representations are in the format as shown below.

YYYY-Www or YYYYWww

YYYY-Www-D or YYYYWwwD

[YYYY] indicates the so-called ISO year which is slightly different than the calendar year (see below). [Www] is the week number prefixed by the letter ‘W’, from W01 through W53. [D] is the weekday number, from 1 through 7, beginning with Monday and ending with Sunday. This form is popular in the manufacturing industries.

There are mutually equivalent definitions for week 01:

• the week with the year’s first Thursday in it,


• the week with 4 January in it,


• the first week with the majority (four or more) of its days in the starting year, and


• the week starting with the Monday in the period 29 December – 4 January.

If 1 January is on a Monday, Tuesday, Wednesday or Thursday, it is in week 01. If 1 January is on a Friday, Saturday or Sunday, it is in week 52 or 53 of the previous year.

The week number can be described by counting the Thursdays: week 12 contains the 12th Thursday