@JVjv 

I don't quite follow the logic here, why 17 hours after dusk? Sunrise will come about twelve hours after sunset, and the schedule related to sunrise will be triggered at this time.

• If you need to keep lighting on at night (after dusk): Turn ON plug after sunset + Turn OFF plug before sunrise.
• If you want to extend the light time of your plants during the day: Turn On plug before sunrise + Turn OFF plug after sunset. 

  @JVjv 

The suggestion I gave above was based on the fact that you could then additionally add offsets to sunset/sunrise. I feel that you may be stuck in the final effect you want to achieve. 

Instead of keeping the plug on for a certain amount of time, you could think it differently. By setting the sunrise/sunset offsets (Before or After), you could make the plug cumulatively be ON for the desired amount of time.

• If you want to extend the light time of your plants during the day: Turn On plug XX hours XX minutes before sunrise + Turn OFF plug XX hours XX minutes after sunset. 

You can be flexible about whether to use Before or After, depending on the situation. For example, in summer, the daytime is longer, sunrises early and the sunset is late, there is no need to extend the light time, then you can change to turn on the light at sunrise, and turn off the light an hour or two before sunset.