Advantages and disadvantages of thermoluminescence dating
The number of weeks in a given year is equal to the corresponding week number of 28 December, because it is the only date that is always in the last week of the year since it is a week before 4 January which is always in the first week of the year.
Using only the ordinal year number, the number of weeks in that year can be determined: Add 3 to the number of this weekday, giving a correction to be used for dates within this year.
In the period 4 January to 28 December the ISO week year number is always equal to the Gregorian year number. The ISO 8601 definition for week 01 is the week with the Gregorian year's first Thursday in it.
The following definitions based on properties of this week are mutually equivalent, since the ISO week starts with Monday: If 1 January is on a Monday, Tuesday, Wednesday or Thursday, it is in week 01.
For an overview of week numbering systems see week number.
The US system has weeks from Sunday through Saturday, and partial weeks at the beginning and the end of the year, i.e. An advantage is that no separate year numbering like the ISO year is needed.
The Gregorian leap cycle, which has 97 leap days spread across 400 years, contains a whole number of weeks ().
In every cycle there are 71 years with an additional 53rd week (corresponding to the Gregorian years that contain 53 Thursdays).
If it is on a Saturday, it is part of the last week of the previous year which is numbered 52 in a common year and 53 in a leap year. This week’s properties are: If 31 December is on a Monday, Tuesday or Wednesday, it is in week 01 of the next year.
The US broadcast calendar counts the week containing 1 January as the first of the year, but otherwise works like ISO week numbering without partial weeks.