Managers and other leaders sometimes assign project goals and deadlines without fully understanding all thay is involved in completing those projects. For you own sanity, it can be helpful to use Casey’s eTip: Negotiate Project Goals When Needed.
When a deadline seems unrealistic, you can politely and respectfully say “yes” and still take care of yourself and maybe your team. Talk with your supervisor about the Scope Triangle: One side is Quality, another Cost and the third Time. Which of the sides is fixed? Whether it’s one or two sides, the remaining side(s) will have to be adjusted. If time is of the essence, for example, quality might go down and cost up. What can the organization live with? What new resources must be added?