[R] Incompatible methods ("+.Date", "Ops.difftime") for "+"

[Richard M. Heiberger]

I received a warning about incompatible methods that I don't understand.
The workaround in this example is simple.

What is the reason that these methods are seen as incompatible?
Is there some dangerous consequence of my as.numeric() workaround?
Can a fix to the code base, perhaps similar to my workaround, make this
warning no longer necessary?

I am using R 2.7.0 on Windows.
version.string R version 2.7.0 (2008-04-22)

Thanks

Rich



xlim <- as.Date(c("2005-04-09", "2007-09-07"))
dx <- diff(xlim)
xlim + c(-.02, .02)*dx              ## correct answer and warning message 
xlim + as.numeric(c(-.02, .02)*dx)  ## correct answer

> xlim <- as.Date(c("2005-04-09", "2007-09-07"))
> xlim
[1] "2005-04-09" "2007-09-07"
> dx <- diff(xlim)
> dx
Time difference of 881 days
> xlim + c(-.02, .02)*dx              ## correct answer and warning message 
[1] "2005-03-22" "2007-09-24"
Warning message:
Incompatible methods ("+.Date", "Ops.difftime") for "+" 
> xlim + as.numeric(c(-.02, .02)*dx)  ## correct answer
[1] "2005-03-22" "2007-09-24"
>