My doctor just told me I'm moderately vitamin d deficient. Can I reverse it by simply spending more time outdoors? How long will it take to be normal?

Possibly. It depends how deficient but i would recommend otc vit d supplement 2000 units daily in addition.
Hypovitaminosis D. Exposure to sunlight is one of the best ways to produce vitamin d.Many factors can affect your production like time and length of exposure, skin color, amount of sunlight available, amount of skin exposed. Dr. Mercola has a good article on thie subject. His website is www.Mercola.Com. Vitamin d deficiency may be associated with over 2000 medical conditions.