chronoptimist: A person who always under estimates the time necessary to do something or get somewhere.

• John: Hey, Cindy. you know my parents are expecting us in 20 minutes.
• Cindy: No problem. I just have to wash the dishes, take a shower, do my hair, walk the dog and then I'm all good to go. See you in 15.
• John: You are such a chronoptimist! I'll see you in 45.
• -- Urban Dictionary