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Why Is MySQL Time Limited to 838:59:59?

An interesting question from Stack Overflow asks “Why is MySQL’s maximum time limit 838:59:59?”

The official reference at http://dev.mysql.com/doc/refman/5.7/en/time.html says

TIME values may range from -838:59:59 to 838:59:59. The hours part may be so large because the TIME type can be used not only to represent a time of day (which must be less than 24 hours), but also elapsed time or a time interval between two events (which may be much greater than 24 hours, or even negative).

The TIME values were always stored on 3 bytes in MySQL. But the format changed on version 5.6.4.

I suspect this was not the first time when it changed. But the other change, if there was one, happened long time ago and there is no public evidence of it. The MySQL source code history on GitHub starts with version 5.5 (the oldest commit is from May 2008) but the change I am looking for happened somewhere around 2001-2002. The first stable release of MySQL 4 (version 4.0.4) was launched in September 2002.

How the TIME values are stored since version 5.6.4

The current format, as described in the documentation, uses the 24 bits as follows:

  • 6 bits for seconds (possible values: 0 to 63);
  • 6 bits for minutes (again, possible values: 0 to 63);
  • 10 bits for hours (possible values: 0 to 1023);
  • 1 bit for sign;
  • 1 bit is unused and labelled “reserved for future extensions”.

It is optimized for working with time components (hours, minutes, seconds) and it doesn’t waste much space. Using this format it’s possible to store values between -1023:59:59 and +1023:59:59. However MySQL limits the number of hours to 838, probably for backward compatibility with applications that were written a while ago, when I think 838 hours was the upper limit.

The format of TIME before version 5.6.4

Until version 5.6.4, the TIME values were also stored on 3 bytes and the components were packed as days * 24 * 3600 + hours * 3600 + minutes * 60 + seconds.
This format was optimized for working with timestamps (because it was, in fact, a timestamp). Using this format it would be possible to store values in the range of about -2330 to +2330 hours.

I think this format was introduced in MySQL 4.

While having this big range of values available, MySQL was still limiting the values to -838 to +838 hours for compatibility with applications developed on MySQL 3.

The format of TIME before MySQL 4

There was bug #11655 on MySQL 4. It was possible to return TIME values outside the -838..+838 range using nested SELECT statements. It was not a feature but a bug and it was fixed.

The only reason to limit the values to this range and to actively change any piece of code that produces TIME values outside it was backward compatibility.

I suspect MySQL 3 used a different format that, due to the way the data was packed, limited the valid values to the range -838..+838 hours.

By looking into the current MySQL’s source code I found this interesting formula:

#define TIME_MAX_VALUE (TIME_MAX_HOUR*10000 + TIME_MAX_MINUTE*100 + TIME_MAX_SECOND)

Let’s ignore for the moment the MAX part of the names used above and let’s remember only that TIME_MAX_MINUTE and TIME_MAX_SECOND are numbers between 00 and 59. The formula just concatenates the hours, minutes and seconds in a single integer number.
For example, the value 170:29:45 becomes 1702945.

This formula raises the following question: given that the TIME values are stored on 3 bytes with sign, what is the maximum positive value that can be represented this way?

The maximum value that can be stored on 3 digits with sign is 0x7FFFFF that in decimal notation is 8388607. Since the last four digits (8607) should be read as minutes (86) and seconds (07) and their maximum valid value is 59, the greatest value that can be stored on 3 bytes with sign using the formula above is 8385959. Which, as TIME is +838:59:59. Ta-da!

Guess what? The fragment of C code listed above was extracted from:

title:my_time.h linenos:1 mark:94 link_text:'See the file on Github' url:https://github.com/mysql/mysql-server/blob/5.7/include/my_time.h#L90
90
91
92
93
94
/* Limits for the TIME data type */
#define TIME_MAX_HOUR 838
#define TIME_MAX_MINUTE 59
#define TIME_MAX_SECOND 59
#define TIME_MAX_VALUE (TIME_MAX_HOUR*10000 + TIME_MAX_MINUTE*100 + TIME_MAX_SECOND)

Now I am sure this is how MySQL 3 used to keep the TIME values internally. This format imposed the limitation of the range, and the backward compatibility requirement on the subsequent versions propagated the limitation to our days.

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