2

I am trying to create a new calculated field, length_of_time. I have a column final_date with dates:

2/10/2016  
4/4/2016  
5/8/2016  
10/1/2016

and I am trying to calculate a new field showing length of time between 10/23/2016 and final_date.

I tried using dplyr:

mutate(df, length_of_time = 10/23/2016 - final_date)

and received an error:

"Error in eval(substitute(expr), envir, enclos) : can only subtract from "POSIXt" objects"

so then I tried to use:

df <- as.POSIXlt(df$final_date)

and again ran my original code, only to receive the following error:

Error in UseMethod("mutate_") : 
  no applicable method for 'mutate_' applied to an object of class "c('POSIXlt', 'POSIXt')"
SymbolixAU
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James Steele
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1 Answers1

5

You've got your date formats a bit confused. (see code comments for explanation)

library(dplyr)

df <- data.frame(final_date = c("2/10/2016","4/4/2016"))

## you need to specify the format of your date columns as it is ambiguous
## I've guessed you're using day/month/year
df$final_date <- as.POSIXct(df$final_date, format = "%d/%m/%Y")

## and you need to subtract the `final_date` (which is POSIXct) 
## from another POSIXct object
mutate(df, length_of_time = as.POSIXct("2016-10-23") - final_date)

final_date length_of_time
1 2016-10-02  20.95833 days
2 2016-04-04 201.95833 days

Further Reading

To help understand the difference between POSIXct and POSIXlt, date formats, date calculations etc.

SymbolixAU
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