Have the possibility to planified a task with a due date including contingency, would be very great.
A task of 3 man/day, planified on 5 days, to let the time to make unplanified support tasks.
So the result would be:
Task 1 - Development: 3 days, planified on 21/05 - 25/05
Task 2 - Maintenance : 2 days, planified on 21/05 - 25/05