iOS计算两个日期的时间间隔，多少天数、周数、月数、年数、时、分、秒
//创建两个日期
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:@"yyyy-MM-dd"];
NSDate *startDate = [dateFormatter dateFromString:@"2017-07-26"];
NSDate *endDate = [dateFormatter dateFromString:@"2021-09-01"];
//利用NSCalendar比较日期的差异
NSCalendar *calendar = [NSCalendar currentCalendar];
/**
* 要比较的时间单位,常用如下,可以同时传：
*
* NSCalendarUnitDay : 天
* NSCalendarUnitYear : 年
* NSCalendarUnitMonth : 月
* NSCalendarUnitHour : 时
* NSCalendarUnitMinute : 分
* NSCalendarUnitSecond : 秒
* NSCalendarUnitWeekdayOrdinal : 星期
*/
NSCalendarUnit unit = NSCalendarUnitWeekdayOrdinal;//只比较天数差异
//比较的结果是NSDateComponents类对象
NSDateComponents *delta = [calendar components:unit fromDate:startDate toDate:endDate options:0];
//打印
NSLog(@"%@",delta);
//获取其中的"天"
NSLog(@"day: %ld",delta.day);
NSLog(@"era: %ld",delta.era);
NSLog(@"year: %ld",delta.year);
NSLog(@"month: %ld",delta.month);
NSLog(@"hour: %ld",delta.hour);
NSLog(@"minute: %ld",delta.minute);
NSLog(@"second: %ld",delta.second);
NSLog(@"weekday: %ld",delta.weekday);
NSLog(@"weekdayOrdinal: %ld",delta.weekdayOrdinal);
