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  2. Determination of the day of the week - Wikipedia

    en.wikipedia.org/wiki/Determination_of_the_day...

    For example, September and December correspond, because 1 September falls on the same day as 1 December (as there are precisely thirteen 7-day weeks between the two dates). Months can only correspond if the number of days between their first days is divisible by 7, or in other words, if their first days are a whole number of weeks apart.

  3. Day count convention - Wikipedia

    en.wikipedia.org/wiki/Day_count_convention

    This convention accounts for days in the period based on the portion in a leap year and the portion in a non-leap year. The days in the numerators are calculated on a Julian day difference basis. In this convention the first day of the period is included and the last day is excluded. The CouponFactor uses the same formula, replacing Date2 by Date3.

  4. Calendar-based contraceptive methods - Wikipedia

    en.wikipedia.org/wiki/Calendar-based...

    The first day of red bleeding is considered day one of the menstrual cycle. To use these methods, a woman is required to know the length of her menstrual cycles. Imperfect use of calendar-based methods would consist of not correctly tracking the length of the woman's cycles, thus using the wrong numbers in the formula, or of having unprotected ...

  5. Estimated date of delivery - Wikipedia

    en.wikipedia.org/wiki/Estimated_date_of_delivery

    280 days past the start of the last menstrual period is found by checking the day of the week of the LMP and adjusting the calculated date to land on the same day of the week. Using the example above, 8 May 2020 is a Friday. The calculated date (15 February) is a Monday; adjusting to the closest Friday produces 12 February, which is exactly 280 ...

  6. Zeller's congruence - Wikipedia

    en.wikipedia.org/wiki/Zeller's_congruence

    These formulas are based on the observation that the day of the week progresses in a predictable manner based upon each subpart of that date. Each term within the formula is used to calculate the offset needed to obtain the correct day of the week. For the Gregorian calendar, the various parts of this formula can therefore be understood as follows:

  7. Doomsday rule - Wikipedia

    en.wikipedia.org/wiki/Doomsday_rule

    Applying the Doomsday algorithm involves three steps: determination of the anchor day for the century, calculation of the anchor day for the year from the one for the century, and selection of the closest date out of those that always fall on the doomsday, e.g., 4/4 and 6/6, and count of the number of days between that date and the date in ...

  8. Julian day - Wikipedia

    en.wikipedia.org/wiki/Julian_day

    The first year of the current Julian period, or that of which the number in each of the three subordinate cycles is 1, was the year 4713 BC, and the noon of January 1 of that year, for the meridian of Alexandria, is the chronological epoch, to which all historical eras are most readily and intelligibly referred, by computing the number of ...

  9. Calendrical calculation - Wikipedia

    en.wikipedia.org/wiki/Calendrical_calculation

    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).