It should be where ever you stored the email you received after you purchased PMD, or if you use a password/license manager like 1Password, in there. In 1Password it should be in the "Software" section.
I just checked my copy of PMD and the 5/4/2014 is most likely the date PMD 2.1.4 was released. The 2017 copyright date is because Nick has created a new company this year and that company now has the copyrights for the previous Catamount Software applications.
User of Jefferson Bible, MPG, PMD, & PMiOS. Oh, and WeatherStick.
Mac OS El Capitán, iOS 10.x.x, watchOS 3.x.x