"date".is.date(x)date - date as a
numeric, and date +- numeric as a date. is.date returns TRUE if x has class
"date", and FALSE otherwise. Its behavior is unaffected
by any attributes of x; for example, x could be a date
array (in contrast to the behavior of is.vector).
as.date returns x if x is a simple object of
class "date", and otherwise a date vector of the same length as
x and with data resulting from coercing the elements of
x to class "date". See the manual page for
as.date() for details.
Logical operations as well as the numeric functions exp(),
log(), and so on are invalid.
Other methods exist for missing value, as.character(),
printing, and summarizing.
date.mdy,
mdy.date,
date.ddmmmyy,
as.date.