R help - Apr 2009 - value of strptime in R 1.8.0

Dear R friends,
I have a data frame, I need to get a time interval between the two columns.
The times are recorded in 24 hour clock. My data frame is called
version.one.
my commands are:
t.s.one<-paste(version.one[,9])
t.s.two<-paste(version.one[,61])
x<-strptime(t.s.one,format="%H:%M")
x
y<-strptime(t.s.two ,format="%H:%M")
y
z<-difftime(y,x, units = "mins")
z

But now in my z object i have negative numbers. Would you please let me know
why? And how can I get rid of this problem?
Thanks
Mitra

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Mitra;

n Apr 12, 2009, at 8:09 PM, Mitra Jazayeri wrote:
> Dear R friends,
> I have a data frame, I need to get a time interval between the two  
> columns.
> The times are recorded in 24 hour clock. My data frame is called
> version.one.
> my commands are:
> t.s.one<-paste(version.one[,9])
> t.s.two<-paste(version.one[,61])
> x<-strptime(t.s.one,format="%H:%M")
> x
> y<-strptime(t.s.two ,format="%H:%M")
> y
> z<-difftime(y,x, units = "mins")
> z
>
> But now in my z object i have negative numbers. Would you please let  
> me know
> why?
Given that neither version.one[,9] nor version[,61] is available to  
us, how can readers of this be expected to answer  other than with the  
trivial hypothesis that t.s.one is after t.s.two?
> And how can I get rid of this problem?
Change the order of the calculation?
(You cannot use abs, since that function is not defined for difftime  
objects.)

-- 
David Winsemius, MD
Heritage Laboratories
West Hartford, CT