EventCalc is a web application that accurately calculates the date of a half birthday along with some potentially interesting numbers like the number of days since the entered date, number of days before the current date and the next half birthday and birthday, etc. It also does the same for anniversaries. It allows people to set up reminders so that they get SMS messages or emails when an important date is approaching and lets them set up a count down (or count up) to or from a date and time.
EventCalc is now in its third incarnation (version 3). It was really terrible when it debuted in the summer of 2007. Back then it was just a way to fool around with building web applications and settle some arguments with friends. Many years later it is a relatively slick single-page application that gets users from all over the world looking up thousands of birthdays and other dates every day. Who would have thought?