how to handle project deadline

Recently found a job at a city government. Everything seems ok except deadline. I am a software developer. Last week worked on a project. Yesterday my manager told me the deadline was next Monday.  I feel wield since I am the person who will do the project, how can she not discuss with me about how long it will take to finish the project. The developer should estimate how long to finish the project. I don't want to offense my boss but I also want to point out the deadline should be set based on my esitmation. Otherwise the deadline is jsut not reasonable. If I don't finish the project before the deadline she has set, that puts me in a bad position. If I cannot finish it before the deadline, then it seems my fault. How can I explain that to my boss?