Your due date, the calulated day when the baby is due, is the most important information you should know. Not because the baby will be born exactly on that day, only 5% of all babies are born on their due date, and 80% are born within 2 weeks before and after the due date. Your due date is important because everything being done during prenatal care and decisions made related to information and interventions relie on the exact due date. If you are off by even 1-2 weeks then mistakes can be made.
I am therefore shocked to find all the time websites that misinform you about the due date like this here.
That website mentions a calculation called the "Naegele's rule" (they even misspell the name of Franz Karl Naegele after whom it was named. Naegele's true rule is to add 7 days then 9 months to the first day of the last menstrual period, but by mistake they say to add 15 days which is incorrect. For example, your last period was April 1,2016 then the correct calculation would give a due date of January 8, 2017.
A much better way to calculate the due date would be to use the BabyMed due date calculator, which makes it much more precise by letting you enter more precise dates like the date of ovulation, or the normal length of your menstrual cycle. The correct due date would be January 5, and not January 16 if you use the incorrect calculation.