When adding a filter on a DataList sometimes you need to do a <= rather than a <. Unfortunately the LessThanOrEqual and GreatherThanOrEqual search filters do not exist in SilverStripe 3.0.x, they were added in 3.1.
Therefore we have to write our own where clause like the following example:
// Return all the sessions on or before the $date
$sessions = new DataList('AcademicSession');
$sessions = $sessions->where('"SessionStartDate" <= \'' . $date . '\'');