What is the best way to turn on 12V light at specific time for 5 minutes?
You are correct that a 555 is not the way to solve this problem. To do this with a microcontroller you need one with that has or interfaces with a real-time clock, some programming (and date and time programs are generally tricky) and some additional hardware to switch the light.


If you want to learn then you could look around at various developer systems such as Arduino, etc.
If you want to solve a problem in a way that someone else can modify it then buy a commercial timer. This can switch the mains input to the 12 V power supply. Costs are as low as &lt; €10, &pound;8, US$12 but watch out for quality.




Figure 1. A programmable plug-in timer. Image source: Amazon.com.