Gasoline once you pump it into your cars gas tank starts to go bad within 30 to 60 days and the longer it sits the worse the gasoline gets.
The gasoline starts to degrade and lose it's octane rating overtime and after 30 to 60 days the gasoline loses quite a bit of octane and the smell and color of the gasoline starts changing.
Once that happens it makes the gasoline less volatile meaning it doesn't ignite as easily in the vehicles engine where it can combust.
The old gasoline can make it hard for your car to start and make it hard for your car to run properly and it can also collect water droplets in the old gasoline.
Gasoline in a vehicle should really be used up within a month or two months at most to prevent the gasoline from going bad.
If you cannot use the gasoline up within a month then just put enough gasoline into the vehicle to run your car as much as you need it and then put more fresh gasoline in the tank when needed.
After 6 months of the gasoline sitting in the tank it usually is turned to varnish and will clog fuel filters, fuel injectors and possibly your fuel pump as well.