Can you choose to heat the room outside of the pre-determined comfort time, ie you have come home early and want heat sooner than the timer has selected?

Yes, at any time, you can increase the temperature on the heater and it will respond, but it will be treated as a temporary override. When the timer rolls onto the next time period in memory, it will then revert to how the timer was set for time & temperature control. 

Applicable models: 

DAR050 DAR075 DAR100 DAR150 DAR200