Found the solution, so answering my own noob question here 
I saved the response from the raw db query as a variable, and then used the js method .map() to return a new array with just the dates:
const rawDates = await strapi.connections.default.raw("SELECT published_at FROM posts WHERE author='1' ORDER BY published_at ASC");
const dates = rawDates.rows.map(row => row.published_at);
return dates;
I understand that querying the db directly like this is better performance wise, but just for learning purposes, what’s the strapi.query equivalent solution… using find/search?