NSDate和NSString互转

//NSDate转NSString
- (NSString *)stringFromDate:(NSDate *)date {
    NSDate *currentDate = date;
    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
    //设置格式：精确到毫秒
    [dateFormatter setDateFormat:@"YYYY-MM-dd hh:mm:ss:SSS" ];
    NSString *currentDateString = [dateFormatter stringFromDate:currentDate];
    return currentDateString;
}

//NSString转NSDate
- (NSDate *)dateFromString:(NSString *)string
{
    NSString *dateString = string;
    NSDateFormatter *formatter = [[NSDateFormatter alloc] init] ;
    [formatter setDateFormat:TIME_MILLISECOND_FORMAT];
    NSDate *date = [formatter dateFromString:dateString];
    return date;
}