TIMESTAMPDIFF calculations are wrong for time differences over ~292 years #10397
Description
We rely on Time.Sub to calculate differences between times in timestampdiff, but this doesn't work for timestamps with a difference greater than 9,223,372,036,854,775,807 nanoseconds, or ~292.47 years. This is because Time.Sub returns a Duration, which is really an int64 representing nanoseconds.
Since MySQL timestamps only go to microsecond precision, we can fix this by converting times to their microsecond values and subtracting them.
mysql> select timestampdiff(microsecond, "0000-01-01", "9999-12-31 23:59:59.999999");
+------------------------------------------------------------------------+
| timestampdiff(microsecond, "0000-01-01", "9999-12-31 23:59:59.999999") |
+------------------------------------------------------------------------+
| 315569433599999999 |
+------------------------------------------------------------------------+
1 row in set (0.00 sec)
The above is the largest timestampdiff value we should be able to support, which should be fine because it fits in an int64. (However, we should check if later dates like 9999-12-31 23:59:59.999999 properly convert to their microsecond values)