A due date calculator can provide a fairly accurate estimate of the expected date of birth, but it's important to note that every pregnancy is unique. In fact, only about 4% of babies are born on that exact day. Most babies, 95.5%, are born in the four weeks around this calculated date, i.e. two weeks before or two weeks after.
All in all, a due date calculator is an extremely useful tool for expectant mothers to provide orientation and help them organise their pregnancy.