Dates

R provides several options for dealing with date and date/time data. The builtin as.Date function handles dates (without times); the contributed package chron handles dates and times, but does not control for time zones; and the POSIXct and POSIXlt classes allow for dates and times with control for time zones. The general rule for date/time data in R is to use the simplest technique possible. Thus, for date only data, as.Date will usually be the best choice. If you need to handle dates and times, without time-zone information, the chron package is a good choice; the POSIX classes are especially useful when time-zone manipulation is important. Also, don't overlook the various “as.” functions (like as.Date and as.POSIXlt) for converting among the different date types when necessary.

Except for the POSIXlt class, dates are stored internally as the number of days or seconds from some reference date. Thus, dates in R will generally have a numeric mode, and the class function can be used to find the way they are actually being stored. The POSIXlt class stores date/time values as a list of components (hour, min, sec, mon, etc.) making it easy to extract these parts.

To get the current date, the Sys.Date function will return a Date object which can be converted to a different class if necessary.