Okey, a good place to start is to read the section for adding custom widgets: http://mydevices.com/cayenne/docs/#custom-widgets
I think that you can read the section and figure it out some major questions. Then here in the community we can help you with writing the code, or give you some advises and etc.
For your purpose, you have to write a python file that reads the time ( I actually for me will use a real time shield, if this will turn some appliance!). Also you have to add your sensor in the file and read the measurements. Further logic is not so hard after that. Then you can send the data and monitor it in Cayenne.
What temp sensor you use?