Some deadlines are non-negotiable, but you usually have the most leeway to move a deadline around before it's been set. Don't take on a two-week task with a one-week deadline unless you're really sure you can get it done. And if your plate is already full, don't hesitate to say no. Most managers would rather hear, "There's no way I can get that done this week, you should find someone else," than, "I was too busy to meet that deadline I accepted last week." Even if you scramble and get it done, it probably won't be your best work.
Another way to work with tough deadlines is to negotiate extra help. For example, "I can get the accounting done, but I'll need Bob to file the documents with the FDA."
You just need to be aware of your capabilities and be reasonable about what you can accomplish. It's far better to be upfront about expectations than to fail to meet them later.