calculating time difference in R

Question

I have a data with more that 3 million records having start.time and end.time as two of the variables. The first 10 obs are as follows:

   start.date start.time   end.date end.time 1  2012-07-13   15:01:32 2012-07-13 15:02:42 2  2012-07-05   18:26:31 2012-07-05 18:27:19 3  2012-07-14   20:23:21 2012-07-14 20:24:11 4  2012-07-29   16:09:54 2012-07-29 16:10:48 5  2012-07-21   14:58:32 2012-07-21 15:00:17 6  2012-07-04   15:36:31 2012-07-04 15:37:11 7  2012-07-22   18:28:31 2012-07-22 18:28:50 8  2012-07-09   21:08:42 2012-07-09 21:09:02 9  2012-07-05   09:44:52 2012-07-05 09:45:05 10 2012-07-02   18:50:47 2012-07-02 18:51:38 

I need to calculate the difference between start.time and end.time.

I used the following code:

mbehave11$diff.time <- difftime(mbehave11$end.time, mbehave11$start.time, units="secs") 

But I am getting this error:

Error in as.POSIXlt.character(x, tz, ...) :    character string is not in a standard unambiguous format In addition: Warning messages: 1: In is.na.POSIXlt(strptime(xx, f <- "%Y-%m-%d %H:%M:%OS", tz = tz)) :   Reached total allocation of 1535Mb: see help(memory.size) 

Answer 1

You must turn your strings into date objects before you can do date/time arithmetic. Try this:

a) Reading your data:

R> dat <- read.table(textConnection("start.date start.time end.date end.time 2012-07-13   15:01:32 2012-07-13 15:02:42 2012-07-05   18:26:31 2012-07-05 18:27:19  2012-07-14   20:23:21 2012-07-14 20:24:11"), header=TRUE)  

b) Working on one observation:

 R>  strptime( paste(dat[,1], dat[,2]), "%Y-%m-%d %H:%M:%S")  [1] "2012-07-13 15:01:32" "2012-07-05 18:26:31" "2012-07-14 20:23:21"  

c) Working on the set, converting to numeric:

 R> as.numeric(difftime(strptime(paste(dat[,1],dat[,2]),"%Y-%m-%d %H:%M:%S"),                         strptime(paste(dat[,3],dat[,4]),"%Y-%m-%d %H:%M:%S")))   [1] -70 -48 -50  R>  

Edit Some seven years later by someone else below.

d) Just to explain the results -70 -48 -50 above take a look at the example row by row:

[2012-07-13 15:01:32] - [2012-07-13 15:02:42] = -70 seconds,   [2012-07-05 18:26:31] - [2012-07-05 18:27:19] = -48 seconds,   [2012-07-14 20:23:21] - [2012-07-14 20:24:11] = -50 seconds 

Answer 2

I think you can use the lubridate package
it has a method called ymd_hms
you can use that to get the time from string: it is much faster for large data set

library(lubridate) dat <- read.table(textConnection("start.date start.time end.date end.time 2012-07-13   15:01:32 2012-07-13 15:02:42 2012-07-05   18:26:31 2012-07-05 18:27:19  2012-07-14   20:23:21 2012-07-14 20:24:11"), header=TRUE) starttime = ymd_hms(paste(dat[,1], dat[,2])) endtime = ymd_hms(paste(dat[,3], dat[,4])) interval = difftime(endtime,starttime,units = "secs") 

or you can just do it in one line, but it takes longer time for big dataset:

difftime(paste(dat[,3], dat[,4]),paste(dat[,1], dat[,2]),units = "secs")