I found a pdf of the manual for your stove. See the link below to download the pdf.
If you press the button that looks like a bell under the clock you can use + an - keys to set the minutes for a reminder timer that won't turn off the oven.
After turning the thermostat to the temp you want and function knob to the setting you want set the cooking time as follows.
You press the button that looks like steam coming off a pan below the clock for 1/2 second .
Then use + an - keys to set the minutes. Every 12 minutes will make a new segment light up on the inside ring so the numbers corospond to hours on the inside ring.
When the time is where you want it, wait 6 to 10 seconds and the cooking and timer will start.
Smeg A42 5 Instruction Manual