One thing your doctor was wrong about is the fact that she told you to go out in the sun early morning and early evening.
I've learned while doing research on acquiring vitamin D through sun exposure, that uvb rays, which you need in order to make vitamin D are there from about 10am to 3 pm
This is the time when dermatologist tell you to NOT to be in the sun to avoid skin cancer, but they are promoting vitamin D deficiency by telling the public this.
As long as you don't stay in the sun too long, moderation is key, don't get excessive sun exposure.
If you're caucasian,
15 minutes is long enough or as long as you can stand and not burn.
Don't put lotions or sunblock on because that's going to stop you from making vitamin D.
And black people have to have 6 times more sun exposure due to the melanin in their skin blocking UVA rays chich cause aging and burning , but the melanin blocks uVB rays as well, so they need to be in the sun longer to make vitamin D,
About an hour or two.
Let us know your new vitamin D levels
Good health to you!