Good afternoon, if we chose a time and we are not there at that time can we still go in once we are there?

Yes, as long as you go to the park that day, Six Flags will honor your reservation. But only for the day you reserved.

Yes you can still go in

As long as you get there AFTER your reservation time, you can get in! I've never been early, so not sure about that, but I've been "late" on multiple occasions and its fine.

I arrived 15 minutes late for my reservation due to traffic and was all good to get in. I dont know if there is a time limit you can be later or not.