Can a 17-year-old date a 20-year-old legally?

Technically yes, but sexual activity is subject to endangering a minor and/or statutory rape, depending on the extent.

It is not illegal to date a minor (person under 18 years old), but it is illegal to have sexual relations with a minor. Even though many of these laws are outdated, overly punitive, and ignore reality, they can be very strict in certain circumstances. Be safe and cautious. So you can date, but the safest approach is to wait for any sexual contact until the 17 year old is 18.