Round time by X hours in R? - r

Question

While doing predicting modeling on timestamped data, I want to write a function in R (possibly using data.table) that rounds the date by X number of hours. E.g. rounding by 2 hours should give this:
"2014-12-28 22:59:00 EDT" becomes "2014-12-28 22:00:00 EDT"
"2014-12-28 23:01:00 EDT" becomes "2014-12-29 00:00:00 EDT"
It's very easy to do when you round by 1 hour - using round.POSIXt(.date, "hour") function.
Writing a generic function, like I'm doing below using multiple if statements, becomes quite ugly however:
d7.dateRoundByHour <- function (.date, byHours) {
if (byHours == 1)
return (round.POSIXt(.date, "hour"))
hh = hour(.date); dd = mday(.date); mm = month(.date); yy = year(.date)
hh = round(hh/byHours,digits=0) * byHours
if (hh>=24) {
hh=0; dd=dd+1
}
if ((mm==2 & dd==28) |
(mm %in% c(1,3,5,7,8,10,12) & dd==31) |
(mm %in% c(2,4,6,9,11) & dd==30)) { # NB: it won't work on 29 Feb leap year.
dd=1; mm=mm+1
}
if (mm==13) {
mm=1; yy=yy+1
}
str = sprintf("%i-%02.0f-%02.0f %02.0f:%02.0f:%02.0f EDT", yy,mm,dd, hh,0,0)
as.POSIXct(str, format="%Y-%m-%d %H:%M:%S")
}
Anyone can show a better way to do that?
(perhaps by converting to numeric and back to POSIXt or some other POSIXt functions?)

Answer 1

Use the round_date function from the lubridate package. Assuming you had a data.table with a column named date you could do the following:
dt[, date := round_date(date, '2 hours')]
A quick example will give you exactly the results you were looking for:
x <- as.POSIXct("2014-12-28 22:59:00 EDT")
round_date(x, '2 hours')

Answer 2

This is actually really easy with just base R. The basic idea for round by "odd lots" that you
scale down by an appropriate scale factor
round down to integer in the downscaled unit
scale back up and re-convert
Or in two R code statements:
R> pt <- as.POSIXct(c("2014-12-28 22:59:00", "2014-12-28 23:01:00 EDT"))
R> pt # just to check
[1] "2014-12-28 22:59:00 CST" "2014-12-28 23:01:00 CST"
R>
R> scalefactor <- 60*60*2 # 2 hours of 60 minutes times 60 seconds
R>
R> as.POSIXct(round(as.numeric(pt)/scalefactor) * scalefactor, origin="1970-01-01")
[1] "2014-12-28 22:00:00 CST" "2014-12-29 00:00:00 CST"
R>
The key last line just does what I outlined: convert the POSIXct to a numeric representation, scales it down, then rounds before scaling back up and converting to a POSIXct again.