R中difftime的结果与excel和timeanddate.com不同

Question

t1 <- strptime("2015-02-17 12:20:00", format = "%Y-%m-%d %H:%M:%S", tz = "America/Chicago")
t2 <- strptime("2015-03-13 15:00:00", format = "%Y-%m-%d %H:%M:%S", tz = "America/Chicago")
as.numeric(abs(difftime(t1, t2, units = "mins", tz = "America/Chicago")))

The above returns 34,660 while Excel and http://www.timeanddate.com/date/duration.html both return 34,720. R recognizes t1 as CST and t2 as CDT due to daylight savings on 3/8/15. I wanted to confirm that R is correct, while these other two sources are not.

Answer 1

Depends if you want to take into account local daylight savings, or if these times are two points in time irrelevant of local shifts. Excel and that site are just taking the exact difference between those two times, discarding daylight savings - eg R can get the same result by changing the timezones to UTC , which doesn't observe daylight saving:

difftime(as.POSIXct("2015-03-13 15:00:00",tz="UTC"),
         as.POSIXct("2015-02-17 12:20:00",tz="UTC"), units="mins")
# Time difference of 34720 mins

If I'm sitting in Chicago with a stopwatch and counting the minutes difference between when a local clock in the pub says "2015-02-17 12:20:00" and then hits "2015-03-13 15:00:00" , I'd count 60 minutes less as the clock got wound forward an hour for daylight savings while I was counting.