There is a similar question I have come across to mine on this page:
Date-time differences between rows in R
Here is a very small snippet of my data:
DT
29/07/12 20:05:01
29/07/12 20:20:59
30/07/12 02:42:08
30/07/12 02:53:17
30/07/12 02:53:18
30/07/12 02:53:19
I would like to do the same thing as this person asked ie calculate time differences (delta time) in R between subsequent rows. The timestamps are stored in a data frame with time as date-time (day/month/year hour:min:sec).
This code was kindly suggested and works most the time, apart from when the time intervals break across days, and then I get huge incorrect numbers (such as 31472469 seconds between 29/07/12 20:20:59 and 30/07/12 02:42:08.
c_time <- as.POSIXlt( mydf$c_time )
c_time <- rev( c_time )
difftime(c_time[1:(length(c_time)-1)] , c_time[2:length(c_time)])
Does anyone have any suggestions?
Thanks!
It is better to be explicit with the format otherwise you have to read really carefully what happens in the default option:
mydf <- read.table(text="c_time
29/07/12 20:05:01
29/07/12 20:20:59
30/07/12 02:42:08
30/07/12 02:53:17
30/07/12 02:53:18
30/07/12 02:53:19", sep=",", row.names=NULL, header=T)
c_time <- as.POSIXlt( mydf$c_time, format="%d/%m/%y %H:%M:%S")
c_time <- rev( c_time )
difftime(c_time[1:(length(c_time)-1)] , c_time[2:length(c_time)])
##Time differences in secs
##[1] 1 1 669 22869 958