[Fix #338] Document behavior or round_date for halfway values

R/round.r

@@ -4,7 +4,12 @@
 #' week, month, quarter, or year.
 #' 
 #' \code{round_date} takes a date-time object and rounds it to the nearest
-#' integer value of the specified time unit.
+#' integer value of the specified time unit. For rounding date-ties which is
+#' exactly halfway between two consecutive units, the convention is to round
+#' up. Note that this is in line with the behavior of R's base
+#' \link[base]{round.POSIXt} function but does not follow the convention of the
+#' base \link[base]{round} function which "goes to the even digit" per IEC
+#' 60559.
 #' 
 #' \code{floor_date} takes a date-time object and rounds it down to the nearest integer 
 #' value of the specified time unit.
@@ -20,6 +25,7 @@
 #'   "second", "minute", "hour", "day", "week", "month", "quarter", or "year."
 #' @return x with the appropriate units floored
 #' @keywords manip chron
+#' @seealso \link[base]{round}
 #' @examples
 #' x <- as.POSIXct("2009-08-03 12:01:59.23")
 #' round_date(x, "second")