To write mtr tests easier, we'll add CAST(x AS INTERVAL DAY_SECOND(N)).
For now it will return a VARCHAR with a specific format:

[+][DD ]hh:mm:ss[.ffffff]

It will be helpful to write tests with functions that now understand long time intervals:

• TIMESTMP(ts, time_interval)
• ADDTIME(ts, time_interval)
• EXTRACT(DAY FROM src)

So in this query:

SELECT

  CAST(1223344.123456 AS INTERVAL DAY_SECOND(6)) AS cast_itds,

  EXTRACT(DAY FROM 1223344.123456) AS d,

  EXTRACT(HOUR FROM 1223344.123456) AS h;

+-------------------+------+------+

| cast_itds         | d    | h    |

+-------------------+------+------+

| 5 02:33:44.123456 |    5 |    2 |

+-------------------+------+------+

it will be clear how the day and the hour value are calculated.