[R] Trouble: Time Difference with difftime

[Jim Trabas]

Hello all!!!

I want to measure the duration of events (given a start and an end time).
The catch is that I require the output in calender days. This means:

02-Jan-2011 00:01:00  minus 01-Jan-2011 23:59:00 should be 1 day (although
the real time difference is only 2 minutes)


My data is the following

head(episode.ct)
[1] "2009-07-13 13:37:20 CEST" "2009-07-14 07:29:20 CEST" "2009-07-14
07:51:20 CEST" "2009-07-14 08:45:20 CEST"
[5] "2009-07-14 09:43:20 CEST" "2009-07-14 10:01:20 CEST"

head(episode.end.ct)
[1] "2009-07-13 13:47:20 CEST" "2009-07-14 07:49:20 CEST" "2009-07-14
08:01:20 CEST" "2009-07-14 08:53:20 CEST"
[5] "2009-07-14 09:51:20 CEST" "2009-07-14 10:11:20 CEST"

attributes(episode.ct)
$class
[1] "POSIXct" "POSIXt" 

$tzone
[1] ""

attributes(episode.end.ct)
$class
[1] "POSIXct" "POSIXt" 

$tzone
[1] ""


When I try :

> difftime(episode.ct-episode.end.ct)

I get

Error in as.POSIXct.default(time1) : 
  do not know how to convert 'time1' to class "POSIXct"


Why is that? Is difftime what I need?


Thank you very much
JT

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