NSString NSDate转换，时间间隔等

@interface NSDate(Conversion)
+ (NSDate *)dateFromString:(NSString *)dateString dateFormat:(NSString *)format;
+ (NSString *)stringFromDate:(NSDate *)date dateFormat:(NSString *)format;
+ (NSInteger)timeBetweenFrom:(NSDate*)fromDateTime
                          to:(NSDate*)toDateTime
                    withUnit:(NSCalendarUnit)unit;
@end

@implementation NSDate(Conversion)

+ (NSInteger)timeBetweenFrom:(NSDate*)fromDateTime
                          to:(NSDate*)toDateTime
                    withUnit:(NSCalendarUnit)unit {
    
    NSDate *fromDate;
    NSDate *toDate;
    NSCalendar *calendar = [NSCalendar currentCalendar];
    [calendar rangeOfUnit:unit startDate:&fromDate interval:NULL forDate:fromDateTime];
    [calendar rangeOfUnit:unit startDate:&toDate interval:NULL forDate:toDateTime];
    NSDateComponents *difference = [calendar components:unit fromDate:fromDate toDate:toDate options:0];
    switch (unit) {
        case NSYearCalendarUnit:
            return [difference year];
        case NSMonthCalendarUnit:
            return [difference month];
        case NSWeekCalendarUnit:
            return [difference week];
        case NSDayCalendarUnit:
            return [difference day];
        case NSHourCalendarUnit:
            return [difference hour];
        case NSMinuteCalendarUnit:
            return [difference minute];
        default:
            NSAssert(NO, @"未定义");
            return -1001;
            break;
    }
}

+ (NSDate *)dateFromString:(NSString *)dateString dateFormat:(NSString *)format {
    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
    [dateFormatter setDateFormat: format];
    NSDate *destDate= [dateFormatter dateFromString:dateString];
    return destDate;
}

+ (NSString *)stringFromDate:(NSDate *)date dateFormat:(NSString *)format {
    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];    
    [dateFormatter setDateFormat:format];
    NSString *destDateString = [dateFormatter stringFromDate:date];
    
    return destDateString;
}
@end