R help - Jun 2008 - Incompatible methods ("+.Date", "Ops.difftime") for "+"

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"
>

You can specify units= to be extra sure:

d <- difftime(Sys.Date() + 1,  Sys.Date(), units = "mins")
as.numeric(d, units = "days")


On Wed, Jun 25, 2008 at 12:23 AM, Richard M. Heiberger <rmh at temple.edu>
wrote:
> 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"
>>
>
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