Je was op zoek naar: isoweeknum (Japans - Engels)

isoweeknum(日付)
isoweeknum(date)

isoweeknum(a1) は a1 が "2005-12-21" であれば 51 を返します。
isoweeknum(a1) returns 51 when a1 is "21st of dec".

isoweeknum() 関数は日付が一年の何周目に位置するかを返します。この関数は iso8601 規格 (週は月曜日で始まり日曜日で終る、一年の最初の木曜日を含む週を一年の最初の週とする) に基づいています。
the isoweeknum() function returns number of the week which the date falls into. note that this function is compliant with the iso8601 standard: a week always begins on a monday, and ends on a sunday. the first week of a year is that week which contains the first thursday of the year.

currentdate, currentdatetime, currenttime, date, date2unix, datedif, datevalue, day, dayname, dayofyear, days, days360, daysinmonth, daysinyear, eastersunday, edate, eomonth, hour, hours, isleapyear, isoweeknum, minute, minutes, month, monthname, months, networkday, now, second, seconds, time, timevalue, today, unix2date, weekday, weeknum, weeks, weeksinyear, workday, year, yearfrac, yearsname
currentdate, currentdatetime, currenttime, date, date2unix, datedif, datevalue, day, dayname, dayofyear, days, days360, daysinmonth, daysinyear, eastersunday, edate, eomonth, hour, hours, isleapyear, isoweeknum, minute, minutes, month, monthname, months, networkday, now, second, seconds, time, timevalue, today, unix2date, weekday, weeknum, weeks, weeksinyear, workday, year, yearfrac, years