On each picture of the adoptable pets it shows a date for when they become adoptable and then below that it shows a rescue date (subject to change) what does the "rescue date" mean?

I think Dawn answered it best

the rescue date means the day animal rescue advocates can rescue. They give the public a chance to give the pets forever homes first. Example: Boxers are often rescued by Boxer Breed organizations who will take them in and try and rehome them through there contacts. Inland Valley tries to give the animals a chance to find a family first. If no one adopts, then they let a rescue adopt. These rescue groups often charge over and above what the shelters adoption fee was. And they may require other stipulations.. Inland Valley simply wants you to adopt, neuter/spay, vaccinate, licence & provide the pet with a good home. ☺

They are dogs whom been on the streets. Then put on adoption.

Rescued date is when the animal was picked up by the Humane Society

The "rescue date" is the date the animal was rescued.