simplify leap_year() to support objects supported by year()

closes tidyverse#703

R/leap-years.r

@@ -16,12 +16,10 @@
 #' leap_year(1900) # FALSE
 #' leap_year(2000) # TRUE
 leap_year <- function(date) {
-  recognized <- recognize(date)
-  if (recognized)
-    year <- year(date)
-  else if (all(is.numeric(date)))
+  if (is.numeric(date)) {
     year <- date
-  else
-    stop("unrecognized date format")
+  } else {
+    year <- year(date)
+  }
   (year %% 4 == 0) & ((year %% 100 != 0) | (year %% 400 == 0))
 }

R/util.r

@@ -15,14 +15,6 @@ match_lengths <- function(x, y) {
   list(x, y)
 }
 
-recognize <- function(x) {
-  recognized <- c("POSIXt", "POSIXlt", "POSIXct", "yearmon", "yearqtr", "Date")
-
-  if (all(class(x) %in% recognized))
-    return(TRUE)
-  return(FALSE)
-}
-
 standardise_date_names <- function(x) {
   dates <- c("second", "minute", "hour", "mday", "wday", "yday", "day", "week", "month", "year", "tz")
   y <- gsub("(.)s$", "\\1", x)