NSDate NSString相互转化

//设置转化格式
NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
[formatter setLocale: [NSLocale currentLocale]];
[formatter setTimeZone: [NSTimeZone localTimeZone]];
[formatter setDateStyle: NSDateFormatterMediumStyle];
[formatter setTimeStyle: NSDateFormatterShortStyle];
[formatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];//HH是24小时制， hh是12小时制, HH:mm:ss里面的 mm 不能大写，否则这个分钟会一直是这个yyyy-MM-dd 里面的MM的值,同时dd和ss都不能大写

//    [formatter setDateFormat:@"yyyy年MM月dd日HH时mm分ss秒"];//这样设置也是可行的

//时间戳转化为时间
NSString *doubleStr = @"1446610423";
NSDate *dateFromDouble = [NSDate dateWithTimeIntervalSince1970: doubleStr.doubleValue];
NSLog(@"dateFromDouble = %@", dateFromDouble);

//时间转化为字符串
NSDate *currentDate = [NSDate date];
NSLog(@"currentDate = %@", currentDate);
NSString *strFromDate = [formatter stringFromDate: currentDate];
NSLog(@"strFromDate = %@", strFromDate);

//时间字符串转化为时间
NSString *dateStr = @"2015-11-27 15:11:19";
NSDate *dateFromStr = [formatter dateFromString: dateStr];
NSLog(@"dateFromStr = %@", dateFromStr);//会出现8小时的时差

//出现8小时时差
//解决办法一
[formatter setTimeZone:[NSTimeZone timeZoneWithAbbreviation:@"UTC"]];
NSDate *correctDate = [formatter dateFromString: dateStr];
NSLog(@"correctDate = %@", correctDate);

//解决办法二
NSTimeZone *sysTimeZone = [NSTimeZone systemTimeZone];
NSInteger interval = [sysTimeZone secondsFromGMTForDate: dateFromStr];
NSDate *localDate = [dateFromStr dateByAddingTimeInterval:interval];
NSLog(@"localDate = %@", localDate);

//解决办法三, 这个不太提倡，只是针对于中国
NSDate *chinaDate = [dateFromStr dateByAddingTimeInterval: 8 * 60 * 60];//加上8个小时的时差
NSLog(@"chinaDate = %@", chinaDate);