I just tried and it worked correctly. Make sure you are using the correct start date field.
If you are activate the DEBUG_LISTQUERY global variable and share the SQL with us.
This occurs in other parts of system, like as advanced search in filters, single filters ...
Because if you try select a especific registry you need to know, date + hours for especification in search field ...
Example, imagine you need search a specific registry in Tasks for a single user. For example task completed today. This is a problem because you need know exactly a hour registry in task for input in search filed
Date = dd-mm-yyyy not returned anythyng
You need put Date = dd-mm-yyyy hh:mm
when you select "today" the query that is constructed has the condition "between today 00:00:00 and today 23:59:00", activate the debug query and you will see it.
this is the condition I see in my tests:
WHERE vtiger_activity.activityid > 0 AND vtiger_crmentity.deleted=0 and vtiger_activity.dtstart between '2018-09-30 00:00:00' and '2018-09-30 23:59:00'