WEEKNUM calculates the week number of the year for the internal date value. The International Standard ISO 8601 has decreed that Monday shall be the first day of the week. A week that lies partly in one year and partly in another is assigned a number in the year in which most of its days lie. That means that week number 1 of any year is the week that contains 4th January.
WEEKNUM は、内部日付番号が、1 年の何週目であるかを返します。