Number of days in the current month using iOS?

You can use the NSDate and NSCalendar classes:

NSDate *today = [NSDate date]; //Get a date object for today's date
NSCalendar *c = [NSCalendar currentCalendar];
NSRange days = [c rangeOfUnit:NSDayCalendarUnit 
                       inUnit:NSMonthCalendarUnit 
                      forDate:today];

today is an NSDate object representing the current date; this can be used to work out the number of days in the current month. An NSCalendar object is then instantiated, which can be used, in conjunction with the NSDate for the current date, to return the number of days in the current month using the rangeOfUnit:inUnit:forDate: function.

days.length will contain the number of days in the current month.

Here are the links to the docs for NSDate and NSCalendar if you want more information.

Swift syntax:

let date = NSDate()
let cal = NSCalendar(calendarIdentifier:NSCalendarIdentifierGregorian)!
let days = cal.rangeOfUnit(.CalendarUnitDay, inUnit: .CalendarUnitMonth, forDate: date)

Swift 3 syntax has changed a bit from Erez's answer:

let cal = Calendar(identifier: .gregorian)
let monthRange = cal.range(of: .day, in: .month, for: Date())!
let daysInMonth = monthRange.count