How to subtract/add days from/to a date?

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?

Solution 1:

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:

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:

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"