I'm trying to set the SLA to minutes instead of hours, or set the ability to have a fraction of an hour (i.e. .5 to denote 30 minutes).
So far I have found only one reference to the unit type in class.ticket.php, and changing this did not allow for the grace period to be set in minutes.
.' ,IF(sla.id IS NULL, NULL, DATE_ADD(ticket.created, INTERVAL sla.grace_period HOUR)) as sla_duedate '