[R] Subtracting POSIXct data/times

[Don MacQueen]

See the help page for the difftime() function, which will tell you 
how to specify the units of the differences.
(when you don't specify, it chooses the units according to some rules)

-Don

At 4:24 PM -0400 6/14/10, James Rome wrote:
>I have two dataframe columns of POXIXct data/times that include seconds.
>I got them into this format using for example
>zsort$ETA <- as.POSIXct(as.character(zsort$ETA), format="%m/%d/%Y %H:%M:%S")
>
>My problem is that when I subtract the two columns, sometimes the
>difference is given in seconds, and sometimes it is given in minutes. I
>don't care which it is, but I need to know which one I will get.
>
>DateTime		ETA
>2010-05-16 02:19:56	2010-05-16 03:46:35
>...
>Browse[1]> mins = zsort$ETA - zsort$DateTime
>Browse[1]> mins
>Time differences in hours
>  [1] 1.444167 2.685000 3.077222 3.210278 3.248056 3.281944 3.281944
>3.360278 3.360278 3.582778 4.574444 5.506111 5.857778 6.150278 6.150278
>6.243056 6.243889 6.248056 6.248611 6.248611 6.356667
>attr(,"tzone")
>
>But sometimes the answer is in seconds.
># make a column with the minutes before landing
>>zsort$MinBeforeLand = zsort$ETA - zsort$DateTime
>>  zsort$MinBeforeLand
>Time differences in secs
>  [1]   -50   136   221   878  1192  2263  3296  3959  4968  5846  8709
>11537 12198 12442 12642 15952 18273 19952 20538
>
>How do I specify the resultant units?
>
>Thanks,
>Jim Rome
>
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-- 
--------------------------------------
Don MacQueen
Environmental Protection Department
Lawrence Livermore National Laboratory
Livermore, CA, USA
925-423-1062