Find out the number of days of a month in R

The Hmisc library has a couple of helpful functions for doing this:

require(Hmisc)
monthDays(as.Date('2010-01-01'))

You can write simple function to do that:

numberOfDays <- function(date) {
    m <- format(date, format="%m")

    while (format(date, format="%m") == m) {
        date <- date + 1
    }

    return(as.integer(format(date - 1, format="%d")))
}

Invoke as:

> date = as.Date("2011-02-23", "%Y-%m-%d")
> numberOfDays(date)
[1] 28
> date # date is unchanged
[1] "2011-02-23"

lubridate package has required function "days_in_month" with respect to leapyears.

Your example:

    date <- as.Date("2011-02-23", "%Y-%m-%d")
    lubridate::days_in_month(date)
    # Feb
    # 28

Leap year (2016):

    date_leap <- as.Date("2016-02-23", "%Y-%m-%d")
    lubridate::days_in_month(date_leap)
    # Feb 
    # 29