[Rd] summary() with argument of class difftime (PR#6780)

[hosking at watson.ibm.com]

Full_Name: J. R. M. Hosking
Version: 1.9.0
OS: Windows 2000
Submission from: (NULL) (129.34.20.23)


> dt<-as.Date("2002-03-04")-as.Date("2001-02-03")
> jstr(dt)
difftime(units:days)(1) 394 
> summary(dt)
Error in Summary.difftime(..., na.rm = na.rm) : 
        sum not defined for difftime objects



When x is of class difftime, either sum(x) should be permitted or summary(x)
should not try to compute the mean.

(R 1.9.0, Windows binary from CRAN)