Your CM921 has 6 built in temperature settings based on time of day:
1 - 6:30 AM
2 - 8:00 AM
3 - 12:00 PM
4 - 2:00 PM
5 - 4:00 PM
6 - 10:30 PM
meaning that at 6:30 AM the themostat will change its temperature setting to setting 1, at 8:00 AM it will change the temperature setting to setting 2, and so on. To schedule temperature settings which best fit you, determine which time periods you would like more or less heat.
For instance, to fit what you requested in your question you could program a temperature you would like to maintain in the morning and afternoon times when you would like it to be on, and in the time periods in between set a lower temperature as follows:
1 - 20C
2 - 16C
3 - 16C
4 - 20C
5 - 20C
6 - 16C
With this example your thermostat would turn on at 6:30 AM as long as the temperature was below 20C and maintain that temperature until 8:00 AM. At 8 it would change its setting to 16C allowing temperature to drop from 8 AM to 2 PM to a lower setting to conserve energy until afternoon time when the setting would change back to 20C from 2 PM to 10:30 and then change back to 16C to save energy throughout the night. The actual temperatures would be at your discretion based on your individual comfort level.
Pages 5 and 6 of the following link will guide you in setting each of the 6 temperature settings.
I hope this helps. Let me know if you have any additional questions.