Since we can't create a new sortable managed property in Office 365 I have mapped ows_DueDate to RefinableDate00.
If I add it to a refiner in the search center it allows me to refine tasks by due date as expected.
But I can't seem to use it as a sorting property when creating a result source.
The (tenant) result source fires the query ContentClass:STS_ListItem_Tasks AssignedTo:{User.Name}
.
The kicker is that it actually changes the results in some small way when switching between asc and desc, just not correctly.