There are two possible reasons why tado° would start heating before the time set in your Smart Schedule:
You enabled Early Start.
You enabled Preheat Before Arrival.
When you enable Early Start, tado° ensures that it reaches and maintains the target temperature by the time set on the Smart Schedule. So it starts heating your room earlier than the time set on your Smart Schedule.
To start heating at the right time, tado° takes many factors into account like current temperature, target temperature, your heating system characteristics, building insulation and so on.
For instance, suppose there is a time block in your Smart Schedule that starts at 7:00 am with a target temperature of 21 °C and you've enabled Early Start. If the temperature at night is, say, 18 °C, then tado° starts heating your home earlier to make sure that the room will be at 21 °C by 7:00 am.
You can disable this in the Smart Schedule settings (gear icon). When Early Start is disabled, tado° only starts heating your room at the scheduled time.
Preheat Before Arrival
If the Auto-Assist Skill is activated and the Preheat Before Arrival is enabled, tado° may start heating even when it is in Away Mode.
To start heating at the right time, tado° takes many factors into account like current temperature, target temperature, your Preheat Before Arrival setting, your heating system characteristics, building insulation and so on.
You can set Preheat Before Arrival to off using the slider in the Away tab of the Smart Schedule. When Preheat Before Arrival is off, tado° maintains the Minimum Away Temperature while in Away Mode and starts heating only when it is back in Home Mode.
You can choose to turn off Preheat Before Arrival for some or all of your rooms.
Please note: If a Home member has manually set tado° to Away or Home Mode, automatic Geofencing and Preheat Before Arrival will be deactivated. To reactivate automatic Geofencing and Preheat Before Arrival, tado° has to be switched back to Auto.