puzzled by math on date-time objects

Denis -
  If you look inside of summary.POSIXct, you'll see the
following:

x <- summary.default(unclass(object), digits = digits, ...)[1:6]

In other words, summary accepts the POSIX object, unclasses it
(resulting in a numeric value representing the number of seconds
since January 1, 1960), performs the operation, and then reassigns
the class.  You can do this basic trick yourself.  Suppose we have a  
vector of dates and want the median:

dates =  
as.POSIXct(c('2009-3-15','2009-2-19','2009-3-20','2009-2-18'))
median(dates)

Error in Summary.POSIXct(c(1235030400, 1237100400), na.rm = FALSE) :
 'sum' not defined for "POSIXt" objects

res = median(as.numeric(dates))
structure(res,class='POSIXct')

[1] "2009-03-02 23:30:00 PST"

  I think it's clear that you can do any arithmetic operation on
dates this way, even if it doesn't make sense:

sum(dates)

Error in Summary.POSIXct(c(1237100400, 1235030400, 1237532400,
1234944000 :
 'sum' not defined for "POSIXt" objects

res = sum(as.numeric(dates))
structure(res,class='POSIXct')

[1] "2126-09-08 23:00:00 PDT"

  I'm quite certain that median.POSIXct will be fixed pretty quickly,
but you can always unclass and reclass to do what you need.

                                                    - Phil






On Tue, 10 Mar 2009, Denis Chabot wrote:

Thanks Phil,

but how does summary() finds the median of the same type of object?  
I would have thought the algorithm used when the vector is even  
would  also require the SUM of the POSIX vector. I am glad of the  
solution you propose, but still puzzled a bit!

Denis
Le 09-03-10 ? 12:39, Phil Spector a ?crit :

Denis -
There is no median method for POSIX objects, although
there is a summary object.  Thus, when you pass a POSIX
object to median, it uses median.default, which contains
the following code:

 if (n%%2L == 1L)
    sort(x, partial = half)[half]
 else sum(sort(x, partial = half + 0L:1L)[half + 0L:1L])/2
So when the length of your POSIX vector is odd, it works, but if  
it's even, it would need to take  the sum of a POSIX
object.  Of course, there is no sum method for POSIX objects,  
since it doesn't make sense.
Right now, it looks like your best bet for a summary of POSIX
objects is
summary(a)['Median']

                                    - Phil Spector
					 Statistical Computing Facility
					 Department of Statistics
					 UC Berkeley
					 spector at stat.berkeley.edu
On Tue, 10 Mar 2009, Denis Chabot wrote:

Hi,
I don't understand the following. When I create a small  
artificial set of date information in class POSIXct, I can  
calculate the mean and the median:
a = as.POSIXct(Sys.time())
a = a + 60*0:10; a
[1] "2009-03-10 11:30:16 EDT" "2009-03-10 11:31:16 EDT"  
"2009-03-10 11:32:16 EDT"
[4] "2009-03-10 11:33:16 EDT" "2009-03-10 11:34:16 EDT"  
"2009-03-10 11:35:16 EDT"
[7] "2009-03-10 11:36:16 EDT" "2009-03-10 11:37:16 EDT"  
"2009-03-10 11:38:16 EDT"
[10] "2009-03-10 11:39:16 EDT" "2009-03-10 11:40:16 EDT"
median(a)
[1] "2009-03-10 11:35:16 EDT"
mean(a)
[1] "2009-03-10 11:35:16 EDT"
But for real data (for this post, a short subset is in object c)   
that I have converted into a POSIXct object, I cannot calculate  
the median with median(), though I do get it with summary():
c
[1] "2009-02-24 14:51:18 EST" "2009-02-24 14:51:19 EST"  
"2009-02-24 14:51:19 EST"
[4] "2009-02-24 14:51:20 EST" "2009-02-24 14:51:20 EST"  
"2009-02-24 14:51:21 EST"
[7] "2009-02-24 14:51:21 EST" "2009-02-24 14:51:22 EST"  
"2009-02-24 14:51:22 EST"
[10] "2009-02-24 14:51:22 EST"
class(c)
[1] "POSIXt"  "POSIXct"
median(c)
Erreur dans Summary.POSIXct(c(1235505080.6, 1235505081.1), na.rm  
= FALSE) :
'sum' not defined for "POSIXt" objects
One difference is that in my own date-time series, some events  
are repeated (the original data contained fractions of seconds).  
But then, why can I get a median through summary()?
summary(c)
                Min.                   1st Qu.                     
Median
"2009-02-24 14:51:18 EST" "2009-02-24 14:51:19 EST" "2009-02-24  
14:51:20 EST"
                Mean                   3rd  
Qu.                      Max.
"2009-02-24 14:51:20 EST" "2009-02-24 14:51:21 EST" "2009-02-24  
14:51:22 EST"
Thanks in advance,
Denis Chabot
sessionInfo()
R version 2.8.1 Patched (2009-01-19 r47650)
i386-apple-darwin9.6.0
locale:
fr_CA.UTF-8/fr_CA.UTF-8/C/C/fr_CA.UTF-8/fr_CA.UTF-8
attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods    
base
other attached packages:
[1] doBy_3.7     chron_2.3-30
loaded via a namespace (and not attached):
[1] Hmisc_3.5-2     cluster_1.11.12 grid_2.8.1       
lattice_0.17-20 tools_2.8.1