Deadlines are something with which we are all very familiar. Everyone is always working under a deadline. Although we have an understanding of the term, it meant something very different at its origin. The term deadline dates back to the Civil War. The first recorded use of the word was found associated with prison camps at Andersonville, GA. The deadline was an invisible, or sometimes marked, line that the prisoners could not cross. Guards were instructed to shoot any prisoners that crossed this line, meaning that anyone to cross the line would be dead.
Over the years, this definition has evolved to mean the latest possible time that something can be completed. While the definition is softer, sometimes the implications can feel similar to the original context. We live in a society driven by deadlines, and missing a deadline can have serious consequences. This is why it is important to be realistic with our workload capabilities.
While it seems like a good idea to try to take on as much as possible, it's better to do less - better and on time. Overcommitting can lead to missed deadlines. Being realistic about how much you can get done is important because it allows you to turn out more consistent and high quality work.
At the same time, arbitrary deadlines can mean that tasks will not get accomplished in a timely manner. Deadlines are important because they keep us on track; they just need to be managed correctly. When we approach deadlines correctly, they can be used as tools to manage productivity.