It seems that MariaDB 5.5 handles NOW() and equality checks against time columns in an unexpected way. Notice:
In MySQL 5.0, 5.5, 5.6, 5.7, and MariaDB 10, this query returns a row when NOW() is a time of day between 06:00 and 19:00. However, on MariaDB 5.5, no row is returned. Changing the query to force casting like this allows the query to work:
It is my guess that MariaDB is casting the NOW() to a string, then comparing against the string equivalent of 'start'. Or perhaps it is casting both fields to DATETIMEs and since the start/stop column omits the date component, MariaDB might be putting some incorrect date into the cast DATETIME.