I'm unsure of what you mean by locked, but a student does have access to assignments from the calendar or from the module. If the assignment has not yet opened yet or as already closed, then no, the student should not be able to do anything more than click the assignment and see the directions. The only exception to this is say you don't have a timer on a quiz. Students can start a quiz and walk away from it. Say the quiz closes Friday, April 24. A student can go in Saturday, April 25 and still see the quiz and questions, but since it is past the available date, it will automatically submit the quiz and not let them complete any more questions.
I hope that helps!