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The basic approach of nearly all of the methods to calculate the day of the week begins by starting from an "anchor date": a known pair (such as 1 January 1800 as a Wednesday), determining the number of days between the known day and the day that you are trying to determine, and using arithmetic modulo 7 to find a new numerical day of the week.
Note: In this algorithm January and February are counted as months 13 and 14 of the previous year. E.g. if it is 2 February 2010 (02/02/2010 in DD/MM/YYYY), the algorithm counts the date as the second day of the fourteenth month of 2009 (02/14/2009 in DD/MM/YYYY format) For an ISO week date Day-of-Week d (1 = Monday to 7 = Sunday), use
A calendrical calculation is a calculation concerning calendar dates. Calendrical calculations can be considered an area of applied mathematics. Some examples of calendrical calculations: Converting a Julian or Gregorian calendar date to its Julian day number and vice versa (see § Julian day number calculation within that article for details).
Since in the Gregorian calendar there are 146,097 days, or exactly 20,871 seven-day weeks, in 400 years, the anchor day repeats every four centuries. For example, the anchor day of 1700–1799 is the same as the anchor day of 2100–2199, i.e. Sunday.
Note that all parameters default to the current date, so for example, the second set of parameters can be left out to calculate elapsed time since a past date: {{Age in years, months, weeks and days |month1 = 1 |day1 = 1 |year1 = 1 }} → 2023 years, 11 months, 2 weeks and 6 days
The day of the week can be easily calculated given a date's Julian day number (JD, i.e. the integer value at noon UT): Adding one to the remainder after dividing the Julian day number by seven (JD modulo 7 + 1) yields that date's ISO 8601 day of the week. For example, the Julian day number of 28 January 2025 is 2460704.
Up to 28 February in the calendar being converted from, add one day less or subtract one day more than the calculated value. Give February the appropriate number of days for the calendar being converted into. When subtracting days to calculate the Gregorian equivalent of 29 February (Julian), 29 February is discounted.
27 week years are 5 days longer than the month years (371 − 366), 6.75%. 44 week years are 6 days longer than the month years (371 − 365), 11%. 70 week years are 2 days shorter than the month years (364 − 366), 17.5%. 259 week years are 1 day shorter than the month years (364 − 365), 64.75%. The table shows the long years in a 400-year ...