Wow now I know the difference between autism and ADHD.
It can be a fine line, there is a lot of overlap.
I personally have both Autism (Asperger Syndrome) and ADD (no major hyperactivity) so I can recognize a lot of traits in both characters in myself.
I do zone out a lot and get hyperfixated on work but I'm also punctual because of that same obsessivness.
I had a lot of issues relating to others and reading social cues as a kid but have been able to learn to understand them as I matured.
I have specialized interests I can get entirely lost in to the point of bodily issues (not using the bathroom, eating, drinking, sleeping, etc)
All of these can be used to give one or the other diagnoses. Getting one is just a question of "Does this qualified doctor believe you meet enough criteria on their secret list?"