NSCalendar *calendar = [NSCalendarcurrentCalendar];

NSRange range = [calendarrangeOfUnit:NSDayCalendarUnitinUnit:NSMonthCalendarUnitforDate:[NSDatedate]];

NSUInteger numberOfDaysInMonth = range.length;

numberOfDaysInMonth 即为当月的天数。更多用法查看NSCalendar， NSDateComponents帮助文档

2. 获取当前年份和月份和天数

NSCalendar *calendar = [NSCalendar currentCalendar];

unsigned unitFlags = NSYearCalendarUnit | NSMonthCalendarUnit |  NSDayCalendarUnit;

NSDateComponents *components = [calendar components:unitFlags fromDate:[NSDate date]];

NSInteger iCurYear = [components year];  //当前的年份

NSInteger iCurMonth = [components month];  //当前的月份

NSInteger iCurDay = [components day];  // 当前的号数