As I understand, it is the UVB rays that created vitamin D. So, you need to catch rays when the UV rays are shining.

The atmosphere (glass and some sunscreens) filter out the UVB rays.

I live in a Northern climate. I go out in the sun at noon during the winter for about 15 minutes.

I really wish the local health care system provided better information on this subject. Since sun exposure can cause cancer they usually tell people to avoid sun and get vitamin D through supplements.