Calculations seem to be off after 2.5 years of use

I have this app and have used it every month since Oct of 2013. I have loved the application and enjoyed the features. I noticed that the calculations are now off giving me only 15-15 days between each cycle. Im usually 28 and very regularly even to the hour. I would so appreciate the developers to fix this little bug. Thanks.

Jessica_goeller about Period Tracker by GP Apps, v9.3.5