Did you know you can do this to a degree with the "Advanced upload file assignment" in 1.8 and above.
As with other assignment types you have the "Available from" option that you can set.
But what makes this assignment type different is that you can choose to hide the description of the task from the student until the available date is reached.
This is great because, even though the student might see a task coming, they have no idea what the task requires until the available date is hit.
This is the only way of doing it inside a standard moodle to date.
Assignment
How to: Assignment show on a specific date and time
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