[Rd] Date class shows Inf as NA; this confuses the use of is.na()

[Greg Minshall]

Martin, et al.,

> I think we should allow 'year' to be "double" instead, and so it
> could also be +Inf or -Inf and we'd nicely cover 
> the conversions from and to 'numeric' -- which is really used
> internally for dates and date-times in  POSIXct.

storing years as a double makes me worry slightly about
----
> year <- 1e50
> (year+1)-year
[1] 0
----
which is not how one thinks of years (or integers) as behaving.

cheers, Greg

ps -- sorry for the ">" overloading!