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  2. Julian day - Wikipedia

    en.wikipedia.org/wiki/Julian_day

    The Julian day is a continuous count of days from the beginning of the Julian period; it is used primarily by astronomers, and in software for easily calculating elapsed days between two events (e.g. food production date and sell by date). [1] The Julian period is a chronological interval of 7980 years, derived from three multi-year cycles: the ...

  3. Day count convention - Wikipedia

    en.wikipedia.org/wiki/Day_count_convention

    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. In general, coupon payments will vary from period to period, due to the differing number of days in the periods.

  4. Calendar date - Wikipedia

    en.wikipedia.org/wiki/Calendar_date

    A calendar date is a reference to a particular day represented within a calendar system. The calendar date allows the specific day to be identified. The number of days between two dates may be calculated. For example, "25 January 2025" is ten days after "15 January 2025". The date of a particular event depends on the observed time zone.

  5. Julian calendar - Wikipedia

    en.wikipedia.org/wiki/Julian_calendar

    This intercalary month was formed by inserting 22 or 23 days after the first 23 days of February; the last five days of February, which counted down toward the start of March, became the last five days of Intercalaris. The net effect was to add 22 or 23 days to the year, forming an intercalary year of 377 or 378 days. [5]

  6. Menstruation - Wikipedia

    en.wikipedia.org/wiki/Menstruation

    The first day of menstrual bleeding is the date used for the last menstrual period (LMP). The typical length of time between the first day of one period and the first day of the next is 21 to 45 days in young women, and 21 to 35 days in adults. [2] [3] The average length is 28 days; one study estimated it at 29.3 days. [10]

  7. ISO week date - Wikipedia

    en.wikipedia.org/wiki/ISO_week_date

    It is the first week with a majority (4 or more) of its days in January. Its first day is the Monday nearest to 1 January. It has 4 January in it. Hence the earliest possible first week extends from Monday 29 December (previous Gregorian year) to Sunday 4 January, the latest possible first week extends from Monday 4 January to Sunday 10 January.

  8. Determination of the day of the week - Wikipedia

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

    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.

  9. Calendar - Wikipedia

    en.wikipedia.org/wiki/Calendar

    A day may consist of the period between sunrise and sunset, with a following period of night, or it may be a period between successive events such as two sunsets. [ 25 ] [ 26 ] The length of the interval between two such successive events may be allowed to vary slightly during the year, or it may be averaged into a mean solar day .