##NSDateFormatter的作用

//NSString * -> NSDate *
- (nullable NSDate *)dateFromString:(NSString *)string;
//NSDate * -> NSString *
- (NSString *)stringFromDate:(NSDate *)date;

##常见的日期格式

http://www.cnblogs.com/mailingfeng/archive/2011/07/28/2120422.html

###NSString * -> NSDate *

"2016-10-03 14:01:00"

// 时间字符串
NSString *string = "2016-10-03 14:01:00";

// 日期格式化类
NSDateFormatter *fmt = [[NSDateFormatter alloc] init];
// 设置日期格式(为了转换成功)
fmt.dateFormat = @"yyyy-MM-dd HH:mm:ss";

// NSString * -> NSDate *
NSDate *date = [fmt dateFromString:string];

NSLog(@"%@", date);

10月-03号/2016年 09-10:05秒

// 时间字符串
NSString *string = @"10月-03号/2016年 09-10:05秒";

// 日期格式化类
NSDateFormatter *fmt = [[NSDateFormatter alloc] init];
fmt.dateFormat = @"MM月-dd号/yyyy年 HH-mm:ss秒";

NSLog(@"%@", [fmt dateFromString:string]);

Tue May 31 17:46:55 +0800 2011

// 时间字符串
NSString *string = @"Tue May 31 17:46:55 +0800 2011";

// 日期格式化类
NSDateFormatter *fmt = [[NSDateFormatter alloc] init];
fmt.dateFormat = @"EEE MMM dd HH:mm:ss Z yyyy";
// fmt.dateFormat = @"EEE MMM dd HH:mm:ss ZZZZ yyyy";
// 设置语言区域(因为这种时间是欧美常用时间)
fmt.locale = [[NSLocale alloc] initWithLocaleIdentifier:@"en_US"];

NSLog(@"%@", [fmt dateFromString:string]);

1745645645645

// 时间戳 : 从1970年1月1号 00:00:00开始走过的毫秒数

// 时间字符串 - 时间戳
NSString *string = @"1745645645645";
NSTimeInterval second = string.longLongValue / 1000.0;

// 时间戳 -> NSDate *
NSDate *date = [NSDate dateWithTimeIntervalSince1970:second];
NSLog(@"%@", date);

##NSCalendar的注意点

#define iOS(version) ([UIDevice currentDevice].systemVersion.doubleValue >= (version))

NSCalendar *calendar = nil;
if ([UIDevice currentDevice].systemVersion.doubleValue >= 8.0) {
    calendar = [NSCalendar calendarWithIdentifier:NSCalendarIdentifierGregorian];
} else {
    calendar = [NSCalendar currentCalendar];
}

NSCalendar *calendar = nil;
if ([NSCalendar respondsToSelector:@selector(calendarWithIdentifier:)]) {
    calendar = [NSCalendar calendarWithIdentifier:NSCalendarIdentifierGregorian];
} else {
    calendar = [NSCalendar currentCalendar];
}

NSDate * -> NSString *

NSDate *date = [NSDate date];

NSDateFormatter *fmt = [[NSDateFormatter alloc] init];
fmt.dateFormat = @"yyyy年MM月dd号 HH:mm:ss";

NSString *string = [fmt stringFromDate:date];

获得日期元素

NSString *string = @"2016-10-03 14:01:00";

NSString *month = [string substringWithRange:NSMakeRange(5, 2)];

NSLog(@"%@", month);
// 时间字符串
NSString *string = @"2016-10-03 14:01:00";

// 日期格式化类
NSDateFormatter *fmt = [[NSDateFormatter alloc] init];
// 设置日期格式(为了转换成功)
fmt.dateFormat = @"yyyy-MM-dd HH:mm:ss";

// NSString * -> NSDate *
NSDate *date = [fmt dateFromString:string];

// 利用NSCalendar处理日期
NSCalendar *calendar = [NSCalendar currentCalendar];
NSInteger month = [calendar component:NSCalendarUnitMonth fromDate:date];
NSInteger hour = [calendar component:NSCalendarUnitHour fromDate:date];
NSInteger minute = [calendar component:NSCalendarUnitMinute fromDate:date];

NSLog(@"%zd %zd %zd", month, hour, minute);
// 时间字符串
NSString *string = @"2016-10-03 14:01:00";

// 日期格式化类
NSDateFormatter *fmt = [[NSDateFormatter alloc] init];
// 设置日期格式(为了转换成功)
fmt.dateFormat = @"yyyy-MM-dd HH:mm:ss";

// NSString * -> NSDate *
NSDate *date = [fmt dateFromString:string];

// 利用NSCalendar处理日期
NSCalendar *calendar = [NSCalendar currentCalendar];

NSCalendarUnit unit = NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay | NSCalendarUnitHour | NSCalendarUnitMinute | NSCalendarUnitSecond;
NSDateComponents *cmps = [calendar components:unit fromDate:date];

// NSLog(@"%zd %zd %zd", cmps.year, cmps.month, cmps.day);
NSLog(@"%@", cmps);
日期比较

// 时间字符串
NSString *createdAtString = @"2016-10-03 14:01:00";
NSDateFormatter *fmt = [[NSDateFormatter alloc] init];
fmt.dateFormat = @"yyyy-MM-dd HH:mm:ss";
NSDate *createdAtDate = [fmt dateFromString:createdAtString];

// 手机当前时间
NSDate *nowDate = [NSDate date];

/**
NSComparisonResult的取值
NSOrderedAscending = -1L, // 升序, 越往右边越大
NSOrderedSame, // 相等
NSOrderedDescending // 降序, 越往右边越小
*/
// 获得比较结果(谁大谁小)
NSComparisonResult result = [nowDate compare:createdAtDate];
if (result == NSOrderedAscending) { // 升序, 越往右边越大
NSLog(@"createdAtDate > nowDate");
} else if (result == NSOrderedDescending) { // 降序, 越往右边越小
NSLog(@"createdAtDate < nowDate");
} else {
NSLog(@"createdAtDate == nowDate");
}
// 时间字符串
NSString *createdAtString = @"2016-10-03 14:01:00";
NSDateFormatter *fmt = [[NSDateFormatter alloc] init];
fmt.dateFormat = @"yyyy-MM-dd HH:mm:ss";
NSDate *createdAtDate = [fmt dateFromString:createdAtString];

// 手机当前时间
// NSDate *nowDate = [NSDate date];

// 获得createdAtDate和nowDate的时间间隔(间隔多少秒)
// NSTimeInterval interval = [nowDate timeIntervalSinceDate:createdAtDate];
NSTimeInterval interval = [createdAtDate timeIntervalSinceNow];
NSLog(@"%f", interval);
NSDateFormatter *fmt = [[NSDateFormatter alloc] init];
fmt.dateFormat = @"yyyy-MM-dd HH:mm:ss";

// 时间字符串
NSString *createdAtString = @"2016-10-03 14:01:00";
NSDate *createdAtDate = [fmt dateFromString:createdAtString];

// 其他时间
NSString *otherString = @"2016-10-03 14:01:00";
NSDate *otherDate = [fmt dateFromString:otherString];

// 获得NSCalendar
NSCalendar *calendar = nil;
if ([NSCalendar respondsToSelector:@selector(calendarWithIdentifier:)]) {
calendar = [NSCalendar calendarWithIdentifier:NSCalendarIdentifierGregorian];
} else {
calendar = [NSCalendar currentCalendar];
}

// 获得日期之间的间隔
NSCalendarUnit unit = NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay | NSCalendarUnitHour | NSCalendarUnitMinute | NSCalendarUnitSecond;
NSDateComponents *cmps = [calendar components:unit fromDate:createdAtDate toDate:otherDate options:0];

NSLog(@"%@", cmps);
条件判断的一些注意点

1.判断一个数组中是否有具体内容
1> 正确
if (array.count) {

}

2> 错误
if (array) {

}

2.判断一个字符串是否有具体内容
1> 正确
if (string.length) {

}

2> 错误
if (string) {

}