CALCULATE

CALCULATE

**Days between two Calender dates calculator** uses two dates, uses two dates, in formats $m_1/d_1/y_1$ and $m_2/d_2/y_2$, such that the second date is greater than the first date, and calculates the difference between these dates. It is is an online calendar tool to calculate how many days are between two dates on the calendar.

It is necessary to follow the next steps:

- Enter two dates in the corresponding formats: $m_1/d_1/y_1$ and $m_2/d_2/y_2$. The months must be in the interval $[1,12]$, the days must be in the interval $[1,31]$ and the years must be in the interval $[1900, 2019]$; The first entered date must be greater than the second entered date.
- Press the
**"CALCULATE"**button to make the computation; - Days between two dates calculator will find the date difference as the number of days between these dates.

The number of days in the date `\text{m/d/y}` is determined by the formula

$$\begin{align}{\rm{Number\;of\;days}}&=365(y-1) + \Big[\frac{y-1}{4}\Big]-\Big[\frac{y-1}{100}\Big] +\Big[\frac{y-1}{400}\Big] + d + \Big[\frac{367m-362}{12}\Big] \\&+\left\{ \begin{array}{ll} 0, & m\leq2 \\ -1, & \hbox{if leap year;} \\ -2, & \hbox{otherwise.} \end{array} \right.\end{align}$$

where `m`, `d` and `y` are month, day, and year, respectively, and square brackets $[\;]$ mean round down to the nearest integer.

$$\begin{align}{\rm{Number\;of\;days}}&=365(y-1) + \Big[\frac{y-1}{4}\Big]-\Big[\frac{y-1}{100}\Big] +\Big[\frac{y-1}{400}\Big] + d + \Big[\frac{367m-362}{12}\Big] \\&+\left\{ \begin{array}{ll} 0, & m\leq2 \\ -1, & \hbox{if leap year;} \\ -2, & \hbox{otherwise.} \end{array} \right.\end{align}$$

where `m`, `d` and `y` are month, day, and year, respectively, and square brackets $[\;]$ mean round down to the nearest integer.

The number of days between two calender dates calculator is developed based on the Gregorian calendar (`365` days for ordinary year or `366` days for leap year divided by `12` months from January to December).
Usually, calculating number of days between two given dates by hand is bit complicated because the number days variation between the Gregorian calender months.

To calculate the exact number of days between two dates we will convert both the dates to number of days and then subtract them. Since February is a specially month, we must be careful.

The number of days in the date `\text{m/d/y}` is number of days in prior years, the number of days in prior months of the current year, and the number of days in the current month. It is determined by the following formula:
$$\begin{align}{\rm{Number\;of\;days}}&=365(y-1) + \Big[\frac{y-1}{4}\Big]-\Big[\frac{y-1}{100}\Big] +\Big[\frac{y-1}{400}\Big] + d + \Big[\frac{367m-362}{12}\Big]
\\&+\left\{
\begin{array}{ll}
0, & m\leq2 \\
-1, & \hbox{if leap year;} \\
-2, & \hbox{otherwise.}
\end{array}
\right.\end{align}$$
where `m`, `d` and `y` are month, day, and year of the date, respectively, and square brackets $[\;]$ mean round down to the nearest integer.
This formula can be found in the book [N Dershowitz, E.M Reingold, Calendrical Calculations: 3rd Edition,
Cambridge University Press, Cambridge, UK, 2007.].

For example, to find the number of days in November 15, 2015, we
compute $365 (2015 - 1) = 735,110$ prior nonleap days, $\Big[\frac{2015-1}{4}\Big]=503$
prior leap days, $\Big[\frac{2015-1}{100}\Big]=20$ prior century
years, $\Big[\frac{2015-1}{400}\Big]=5$ prior 400-multiple years, $\Big[\frac{367\times11-362}{12}\Big]=306$ prior days, corrected by $2$ because November is beyond February and 2015 is not a leap year. Adding these results and the day $15$ together gives

$$735,110 + 503 - 20 + 5 + 306 - 2 + 15 = 735,917\mbox{-days}.$$

The days between two calender dates work with steps shows the complete step-by-step calculation for dates `\text{4/30/2018}` and `\text{4/30/2019}`. For any other dates, just supply the dates in format $m_1/d_1/y_1$ and $m_2/d_2/y_2$ and click on the "CALCULATE" button. The grade school students may use this days between two calender dates calculator to find the day difference between two special dates.

In real world, usually we want to find the period between two dates. For instance, period between two salaries or time left until the next delivery. Days between two dates calculator can help us to know the time remaining to special events or how many days are between two events. For instance, it can be useful for calculating how many days are left before a deadline, the number of days until retirement, for determining the time frame of a project, but here we should pay attention to remove weekend and holiday days from the total, etc.

The days difference between two dates calculator and example calculation (work with steps) would be very useful for grade school students (K-12 education) to find the exactly days between two calender dates. By help of it, they will be able to estimate the length of a period.