ExtensionsDate.swift

extension Date {
    var year:    Int { return Calendar.autoupdatingCurrent.component(.year,   from: self) }
    var month:   Int { return Calendar.autoupdatingCurrent.component(.month,  from: self) }
    var day:     Int { return Calendar.autoupdatingCurrent.component(.day,    from: self) }
    var firstDayOfMonth: Date {
        return  Calendar.autoupdatingCurrent.date(from: DateComponents(year: year, month: month, day: 1))!
    }
    var lastDayOfMonth: Date {
        return  Calendar.autoupdatingCurrent.date(from: DateComponents(year: year, month: month+1, day: 0))!
    }
}

Date().firstDayOfMonth // "Dec 1, 2016, 12:00 AM"
Date().lastDayOfMonth // "Dec 31, 2016, 12:00 AM"