1) Create an assignment (see Creating an assignment with attachments) then instead of clicking "Assign", click the little triangle to the right of it.
2) This will give you the options of assigning, scheduling, or saving the assignment as a draft. Click "Schedule".
3) This will prompt you for a date and time. Enter the ones you want and then click "Schedule".
4) This will add the assignment to the Classwork page but you will notice that it is greyed out and will show the date and time it's scheduled for.