I have data that is collected in two different time zones z$OnDateTime <- as.POSIXct(runway$OnDateTime, tz = "EST5EDT", format="%m/%d/%Y %H:%M") is in Eastern time, and zi$ActualOnLocal <- as.POSIXct(oooi$ActualOnLocal, tz="MST7MDT", format="%m/%d/%Y %H:%M") is in Mountain". I converted the runway time to Mountain (I think) z$OnDateTime = format(runway$OnDateTime, tz="MST7MDT") The corresponding times in the two data sets seem identical (within a few minutes) when printed out: ]> z$ActualOnLocal[1:5] [1] "2008-11-01 07:49:00 MDT" "2008-11-03 07:58:00 MST" [3] "2008-11-04 08:04:00 MST" "2008-11-05 07:44:00 MST" [5] "2008-11-06 07:32:00 MST"> z$OnDateTime[1:5][1] "2008-11-01 07:49:00" "2008-11-03 07:57:00" "2008-11-04 08:04:00" [4] "2008-11-05 07:43:00" "2008-11-06 07:32:00" But when I do z$dt= as.numeric(difftime(z$ActualOnLocal, z$OnDateTime, units="mins")) the difference is 120 minutes. How do I get my date times so that difftime works properly? Thanks, Jim Rome