It is way better to be told your table at a restaurant won’t be ready for 60 minutes and then be seated in 50 minutes than it is to be told you will be seated in 30 minutes and you get seated in 40 minutes.
When you communicate a deadline, you are creating an expectation in the minds of other people that they believe you are committed to delivering. When you miss that expectation, people think you are not committed to them.
That creates huge problems in relationships, especially business relationships. People stop trusting you. People like co-workers, bosses, employees, customers, and suppliers.
Build into your deadline the likely possibility that a lot of things will go wrong. It’s better to let people know a later date that you actually meet than an earlier date you consistently miss.