We make use of the dota 2 public GSI to set the reminders, those are public and provided by valve (also used by Overwolf). Additionally, all the code is open, so you can check and modify it by yourself.
By making it open-source we have a safe space to develop the set of tools that we need and actually use. Even if you are not a programmer you can help by raising ideas and suggestions on the github issue tab.
We have provided detailed documentation as well as instructions to help new users get started. We are also always open to feedback and can answer any questions a user may have about Dota 2 Reminders.