I'm guessing you don't have the ability to control the door, just monitor it's status. So, to that end, you'll be able to do this very simply as follows (yes, it's going to take a little work to set up, but the experience will be WELL worth it):
- You'll need to open up the sensor's "Device" page and go to Advanced. Click on the Variables tab. Scroll down through the list and determine which one of them will show you whether the door is open or closed. It might be "Status", for example.
- Create a scene that runs every day at a set time. This will be the time that you want to check the door to see if it's open.
- Write LUA / LUUP code to poll the sensor, retrieve the value of the setting determined in step 1, and return a "TRUE" if it's set to the value that indicates that the door is open.
- Set the scene to alert you when it runs.
- Name it "Garage Door Still Open".
What will happen is that the scene will run every day at a specific time. It will poll the sensor to get its current state. If the value returned indicates that the door is open, you will get an alert.