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Fixed-Duration Date Offsets with Year and Month Approximations

Calculate fixed-duration date offsets with year and month approximations with calendar inputs, retained inverse relationships and explicit duration conventions.

  • Formula & worked example
  • Private in your browser
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Calculator inputs

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

Private by default

Inputs and results stay in this browser tab. Bookify does not upload or store the values you enter.

Formula and method

Adding two elapsed days to 2024-02-28 reaches 2024-03-01. The separate year/month inputs use fixed average lengths.

date2 = date1 + add_or_subtract * (years * 365.25 + months * 30.44 + weeks * 7 + days)

Worked example

Enter these known values and leave the other values blank.

Start date
2024-02-28
Day offset
2
End date
2024-03-01

Assumptions and limitations

  • Civil dates use the proleptic Gregorian calendar from year 0001 through 9999. A selected date represents that calendar day, without a birth time or time zone.
  • Calendar-day arithmetic is independent of daylight-saving clock changes. Weekends and holidays remain included.
  • In this particular captured offset formula, a year is fixed at 365.25 days and a month at 30.44 days. This is not anniversary or end-of-month calendar addition.
  • Calculated dates display the UTC calendar day containing the derived timestamp. Fractional-day offsets can contain a time of day that is not shown in this date-only interface.

Common questions

Does this tool skip weekends and holidays?

It counts calendar days. Weekends and holidays are included; a business-day calendar is a different model.

Are month and year results calendar anniversaries?

In this particular captured offset formula, a year is fixed at 365.25 days and a month at 30.44 days. This is not anniversary or end-of-month calendar addition.

References

The calculation equations, inverse formulas, units, and input rules were imported from this source. Bookify provides the interface and equation solver.

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