Cooking chicken will not extend the use by date.
If you cook the chicken you can keep the cooked chicken in the refrigerator for up to 3 to 4 days before it goes bad.
If you cook the chicken and then freeze it the frozen cooked chicken should last for 2 to 3 months before it goes bad.
When you want to keep the chicken past it's use by date you should freeze the chicken instead and when frozen the chicken will last for up to a year.
Raw chicken lasts only 3 days in the refrigerator and raw chicken lasts up to a year in the freezer.
However the longer the chicken remains in the freezer the less quality it gets and it starts to lose it's best taste.
So it's best to eat the chicken before a year even when frozen.