Number of Months Between Two Dates

Difference in Months between two dates in JavaScript

The definition of "the number of months in the difference" is subject to a lot of interpretation. :-)

You can get the year, month, and day of month from a JavaScript date object. Depending on what information you're looking for, you can use those to figure out how many months are between two points in time.

For instance, off-the-cuff:

function monthDiff(d1, d2) {
    var months;
    months = (d2.getFullYear() - d1.getFullYear()) * 12;
    months -= d1.getMonth();
    months += d2.getMonth();
    return months <= 0 ? 0 : months;
}

function monthDiff(d1, d2) {    var months;    months = (d2.getFullYear() - d1.getFullYear()) * 12;    months -= d1.getMonth();    months += d2.getMonth();    return months <= 0 ? 0 : months;}
function test(d1, d2) {    var diff = monthDiff(d1, d2);    console.log(        d1.toISOString().substring(0, 10),        "to",        d2.toISOString().substring(0, 10),        ":",        diff    );}
test(    new Date(2008, 10, 4), // November 4th, 2008    new Date(2010, 2, 12)  // March 12th, 2010);// Result: 16
test(    new Date(2010, 0, 1),  // January 1st, 2010    new Date(2010, 2, 12)  // March 12th, 2010);// Result: 2
test(    new Date(2010, 1, 1),  // February 1st, 2010    new Date(2010, 2, 12)  // March 12th, 2010);// Result: 1

Pandas - Number of Months Between Two Dates

Here is a very simple answer my friend:

df['nb_months'] = ((df.date2 - df.date1)/np.timedelta64(1, 'M'))

and now:

df['nb_months'] = df['nb_months'].astype(int)

Difference in months between two dates

Assuming the day of the month is irrelevant (i.e. the diff between 2011.1.1 and 2010.12.31 is 1), with date1 > date2 giving a positive value and date2 > date1 a negative value

((date1.Year - date2.Year) * 12) + date1.Month - date2.Month

Or, assuming you want an approximate number of 'average months' between the two dates, the following should work for all but very huge date differences.

date1.Subtract(date2).Days / (365.25 / 12)

Note, if you were to use the latter solution then your unit tests should state the widest date range which your application is designed to work with and validate the results of the calculation accordingly.

Update (with thanks to Gary)

If using the 'average months' method, a slightly more accurate number to use for the 'average number of days per year' is 365.2425.

How can I calculate the number of months between two given dates ( baseline and follow-up)

Here is an option with lubridate

library(dplyr)
library(lubridate)
df1 %>%
      mutate(Months_difference = (interval(mdy(Baseline), 
             mdy(Follow_Up))) %/% months(1))

How do I calculate the number of months between two dates in a specific year

Assumptions

Your start date and months are both strings in separate columns.
You can convert them to actual dates in a helper column.
Data is layed out per image below.

General

I am just laying out my solution in a simplified method. You can in turn nest formulas, hard code items, etc to suit your needs

Process

Convert your start date from a string in A6 to an actual excel date and place it in D6. I simply used the following formula, but other methods exist and may be needed depending on system settings.

=DATEVALUE(A6)

Calculate the end date based on month offset and place it in E6. To achieve this I used the following formula which is different then DATEDIFF which you seem to understand.

=EOMONTH(D6,LEFT(B6,FIND(" ",B6)-1)-2)+1

Place the given year you are looking for in a designated cell. In this case I chose F1.

Based on the integer that is placed in F1 generate an actual date for the first month of the year and for the last month of the year. I placed these in F2 and F3 respectively.

=DATE(F1,1,1)

=DATE(F1,12,1)

Note:  These can easily be embedded in formulas but can be cleaner and make formulas easier to read.

Next you have 6 conditions to test for:

The entire period is before the given year => 0
The entire period is after the given year => 0
The period starts before the given year and finishes after the end of the given year => 12
The period starts before the given year and finishes within the given year => formula 1
The period starts within the given year and finishes after the given year => formula 2
The period starts within the given year and finishes within the given year = formula 3

I am going to approach this with a nested IF and will explain it in steps. Condition 1 and 2 both result in 0. Therefore check if either condition is true and return 0 if so:

=IF(OR(E6<$F$2,D6>$F$3),0,

Note the formula is not finished. The false part of the if was not supplied and its where we can check condition 3.

=IF(OR(E6<$F$2,D6>$F$3),0,IF(AND(D6<=$F$2,E6>=$F$3),12,

So this checks if the Start and end date span the current year. Note I use <= and >= because if the period start date is equal to the start of the current year and or the end date is equal to the end of the current year its the same result and saves going through a math calculation later. Again the false side has not been provided. To fill this part in, you need to check condition 4.

Check to see if the period start date is before the given year. You do not need to check the end date at the same time because if the end date is after you know it was already caught by your previous condition 3 check. If it is, the month of the end date will tell you how many months were in the given year. The month of the end of the period calculation is formula 1

=IF(OR(E6<$F$2,D6>$F$3),0,IF(AND(D6<=$F$2,E6>=$F$3),12,IF(D6<=$F$2,MONTH(E6),

Now you now the next condition check already knows you period start date is after the start of the year otherwise it would have already been caught by one of the previous checks. what you need is to determine if then end of the period is to the end of the year, and calculate the number of month from start of period to end of year which is formula 2.

=IF(OR(E6<$F$2,D6>$F$3),0,IF(AND(D6<=$F$2,E6>=$F$3),12,IF(D6<=$F$2,MONTH(E6),IF(E6>=$F$3,12-MONTH(D6)+1,

There are no further conditions to check as you have caught all the possibilities except one, the final condition. All that needs to be determined is is formula 3 and close all the brackets for your nested IFs.

=IF(OR(E6<$F$2,D6>$F$3),0,IF(AND(D6<=$F$2,E6>=$F$3),12,IF(D6<=$F$2,MONTH(E6),IF(E6>=$F$3,12-MONTH(D6)+1,MONTH(E6)-MONTH(D6)+1))))

Place the formula above in G6 and copy down as required.

This process is not the only way just the first one I went to. Others may have a more elegant solution.

POC