How to subtract/add days from/to a date?
RDateR FaqR Problem Overview
I'm trying to build folders to store data pulls. I want to label the folders with the day of that data in the pull.
Ex. I pull 5 days ago data from mysql i want to name the folder the date from 5 days ago.
MySQL can easily handle date arithmetic. I'm not sure exactly how R does it. Should i just subtract the appropriate number of seconds in POSIXct and then convert to POSIXlt to name the folder MM_DD_YYYY?
Or is there a better way?
R Solutions
Solution 1 - R
Just subtract a number:
> as.Date("2009-10-01")
[1] "2009-10-01"
> as.Date("2009-10-01")-5
[1] "2009-09-26"
Since the Date class only has days, you can just do basic arithmetic on it.
If you want to use POSIXlt for some reason, then you can use it's slots:
> a <- as.POSIXlt("2009-10-04")
> names(unclass(as.POSIXlt("2009-10-04")))
[1] "sec" "min" "hour" "mday" "mon" "year" "wday" "yday" "isdst"
> a$mday <- a$mday - 6
> a
[1] "2009-09-28 EDT"
Solution 2 - R
The answer probably depends on what format your date is in, but here is an example using the Date class:
dt <- as.Date("2010/02/10")
new.dt <- dt - as.difftime(2, unit="days")
You can even play with different units like weeks.
Solution 3 - R
There is of course a lubridate solution for this:
library(lubridate)
date <- "2009-10-01"
ymd(date) - 5
# [1] "2009-09-26"
is the same as
ymd(date) - days(5)
# [1] "2009-09-26"
Other time formats could be:
ymd(date) - months(5)
# [1] "2009-05-01"
ymd(date) - years(5)
# [1] "2004-10-01"
ymd(date) - years(1) - months(2) - days(3)
# [1] "2008-07-29"