Sys.time returns an absolute date-time value which can be converted to various time zones and may return different days.
Sys.Date returns the current day in the current timezone.
Sys.time returns an object of class
"POSIXct" (see DateTimeClasses). On almost all systems it will have sub-second accuracy, possibly microseconds or better. On Windows it increments in clock ticks (usually 1/60 of a second) reported to millisecond accuracy.
Sys.Date returns an object of class
"Date" (see Date).
Sys.time may return fractional seconds, but they are ignored by the default conversions (e.g. printing) for class
"POSIXct". See the examples and
format.POSIXct for ways to reveal them.
date for the system time in a fixed-format character string.
Documentation reproduced from R 3.0.1. License: GPL-2.