If x is a recognized date-time object, leap_year will return whether x occurs during
a leap year. If x is a number, it is interpreted as a year in the Gregorian calendar
and leap_year returns TRUE if it is a leap year.
Examples
x <- as.Date("2009-08-02")
leap_year(x) # FALSE
#> [1] FALSE
leap_year(2009) # FALSE
#> [1] FALSE
leap_year(2008) # TRUE
#> [1] TRUE
leap_year(1900) # FALSE
#> [1] FALSE
leap_year(2000) # TRUE
#> [1] TRUE
