SAD vs. PTSD - Any defining differences?

I've only ever had psychiatric hospitalizations/ER visits/etc. in the range of months from September to January. February has been really bad this year and in the past, but nothing with a paper trail. I have mental health issues year-round and have since I was young, but for whatever reason it's a bit less debilitating in the other parts of the year. I have vicious mental breakdowns in the season we're in now and often go into these totally frozen states of despair-induced paralysis, more so than I do in other parts of the year.

I've never been diagnosed with SAD but I've been in and out of therapy for over a decade and it's something I always get asked about. I am diagnosed with c-PTSD. Is there a way to tell if your seasonal issues (autumnal/winter months, in my case) are related to SAD as opposed to trauma anniversaries in complex post-traumatic stress? This may seem like an obvious answer, but I actually don't remember most of my life so I really don't know what happened where or if it happened at all. I live in a colder climate now, but I used to live somewhere very warm, if that helps. I'm not asking for a diagnosis (obviously), just wondering if there's any information I don't have that would help me figure my own stuff out.

SAD treatment isn't harmful to people who don't have it so if you even suspect it has any influence on your mental health, I don't see any reason not to try it? I don't think there's a way to tell without experimenting.

Also I'm not sure if this is common knowledge or not, or even relevant in this case, but people with dark skin don't get enough vitamin D from the sun alone in colder climates and it's generally suggested there that they take supplements for it all year round since it can already give an average person symptoms of depression if they don't.