Add a month to a Date [duplicate]

Question

This question already has an answer here:

• How to subtract months from a date in R? 2 answers

I am trying to add a month to a date i have. But then its not possible in a straight manner so far. Following is what i tried.

d <- as.Date("2004-01-31")
d + 60
# [1] "2004-03-31"

Adding wont help as the month wont be overlapped.

seq(as.Date("2004-01-31"), by = "month", length = 2) 
# [1] "2004-01-31" "2004-03-02"

Above might work , but again its not straight forward. Also its also adding 30 days or something to the date which has issues like the below

seq(as.Date("2004-01-31"), by = "month", length = 10) 
#  [1] "2004-01-31" "2004-03-02" "2004-03-31" "2004-05-01" "2004-05-31" "2004-07-01" "2004-07-31" "2004-08-31" "2004-10-01" "2004-10-31"

In the above , for the first 2 dates , month haven’t changed.

Also the following approach also failed for month but was success for year

d <- as.POSIXlt(as.Date("2010-01-01"))
d$year <- d$year +1
d
# [1] "2011-01-01 UTC"
d <- as.POSIXlt(as.Date("2010-01-01"))
d$month <- d$month +1
d

Error in format.POSIXlt(x, usetz = TRUE) : invalid 'x' argument

What is the right method to do this ?

Answer 1

It is ambiguous when you say "add a month to a date".

Do you mean

1. add 30 days?
2. increase the month part of the date by 1?

In both cases a whole package for a simple addition seems a bit exaggerated.

For the first point, of course, the simple + operator will do:

d=as.Date('2010-01-01') 
d + 30 
#[1] "2010-01-31"

As for the second I would just create a one line function as simple as that (and with a more general scope):

add.months= function(date,n) seq(date, by = paste (n, "months"), length = 2)[2]

You can use it with arbitrary months, including negative:

add.months(d, 3)
#[1] "2010-04-01"
add.months(d, -3)
#[1] "2009-10-01"

Of course, if you want to add only and often a single month:

add.month=function(date) add.months(date,1)
add.month(d)
#[1] "2010-02-01"

If you add one month to 31 of January, since 31th February is meaningless, the best to get the job done is to add the missing 3 days to the following month, March. So correctly:

add.month(as.Date("2010-01-31"))
#[1] "2010-03-03"

In case, for some very special reason, you need to put a ceiling to the last available day of the month, it's a bit longer:

add.months.ceil=function (date, n){

  #no ceiling
  nC=add.months(date, n)

  #ceiling
  day(date)=01
  C=add.months(date, n+1)-1

  #use ceiling in case of overlapping
  if(nC>C) return(C)
  return(nC)
}

As usual you could add a single month version:

add.month.ceil=function(date) add.months.ceil(date,1)    

So:

  d=as.Date('2010-01-31')
  add.month.ceil(d)
  #[1] "2010-02-28"
  d=as.Date('2010-01-21')
  add.month.ceil(d)
  #[1] "2010-02-21"

And with decrements:

  d=as.Date('2010-03-31')
  add.months.ceil(d, -1)
  #[1] "2010-02-28"
  d=as.Date('2010-03-21')
  add.months.ceil(d, -1)
  #[1] "2010-02-21"

Besides you didn't tell if you were interested to a scalar or vector solution. As for the latter:

add.months.v= function(date,n) as.Date(sapply(date, add.months, n), origin="1970-01-01")

Note: *apply family destroys the class data, that's why it has to be rebuilt. The vector version brings:

d=c(as.Date('2010/01/01'), as.Date('2010/01/31'))
add.months.v(d,1)
[1] "2010-02-01" "2010-03-03"

Hope you liked it))

Answer 2

Here's a function that doesn't require any packages to be installed. You give it a Date object (or a character that it can convert into a Date), and it adds n months to that date without changing the day of the month (unless the month you land on doesn't have enough days in it, in which case it defaults to the last day of the returned month). Just in case it doesn't make sense reading it, there are some examples below.

Function definition

addMonth <- function(date, n = 1){
  if (n == 0){return(date)}
  if (n %% 1 != 0){stop("Input Error: argument 'n' must be an integer.")}

  # Check to make sure we have a standard Date format
  if (class(date) == "character"){date = as.Date(date)}

  # Turn the year, month, and day into numbers so we can play with them
  y = as.numeric(substr(as.character(date),1,4))
  m = as.numeric(substr(as.character(date),6,7))
  d = as.numeric(substr(as.character(date),9,10))

  # Run through the computation
  i = 0
  # Adding months
  if (n > 0){
    while (i < n){
      m = m + 1
      if (m == 13){
        m = 1
        y = y + 1
      }
      i = i + 1
    }
  }
  # Subtracting months
  else if (n < 0){
    while (i > n){
      m = m - 1
      if (m == 0){
        m = 12
        y = y - 1
      }
      i = i - 1
    }
  }

  # If past 28th day in base month, make adjustments for February
  if (d > 28 & m == 2){
      # If it's a leap year, return the 29th day
      if ((y %% 4 == 0 & y %% 100 != 0) | y %% 400 == 0){d = 29}
      # Otherwise, return the 28th day
      else{d = 28}
    }
  # If 31st day in base month but only 30 days in end month, return 30th day
  else if (d == 31){if (m %in% c(1, 3, 5, 7, 8, 10, 12) == FALSE){d = 30}}

  # Turn year, month, and day into strings and put them together to make a Date
  y = as.character(y)

  # If month is single digit, add a leading 0, otherwise leave it alone
  if (m < 10){m = paste('0', as.character(m), sep = '')}
  else{m = as.character(m)}

  # If day is single digit, add a leading 0, otherwise leave it alone
  if (d < 10){d = paste('0', as.character(d), sep = '')}
  else{d = as.character(d)}

  # Put them together and convert return the result as a Date
  return(as.Date(paste(y,'-',m,'-',d, sep = '')))
}

Some examples

Adding months

> addMonth('2014-01-31', n = 1)
[1] "2014-02-28"  # February, non-leap year
> addMonth('2014-01-31', n = 5)
[1] "2014-06-30"  # June only has 30 days, so day of month dropped to 30
> addMonth('2014-01-31', n = 24)
[1] "2016-01-31"  # Increments years when n is a multiple of 12 
> addMonth('2014-01-31', n = 25)
[1] "2016-02-29"  # February, leap year

Subtracting months

> addMonth('2014-01-31', n = -1)
[1] "2013-12-31"
> addMonth('2014-01-31', n = -7)
[1] "2013-06-30"
> addMonth('2014-01-31', n = -12)
[1] "2013-01-31"
> addMonth('2014-01-31', n = -23)
[1] "2012-02-29"