The US Navy developed evaluation and review technique specifically to solve the first problem. For a particular task you can say: optimistic time = 1 week, most likely time = 2 weeks, pessimistic time = 20 weeks. If you decompose a large project into a large number of smaller tasks then the critical path schedule estimate should be close to reality. Or if you want to be more sophisticated you can do a Monte Carlo analysis to create a probability distribution of possible completion dates, and then make commitments based on whatever statistical level of confidence makes you comfortable.