When calculating harvest time does one not consider when the seeds were started? Or is it truely from when one transplants the seedlings?
The DTM(days to maturity) depends on a lot of things. When you see something like 55 days from maturity. It means that SOMEBODY got a ripe tomato in THEIR field in a PARTICULAR year 55 days after they planted it. The days to maturity is based on a "Standard" transplant age and growing conditions.
For example I planted some Silvery Fir tree last year at the end of may beginning of June. Two of them sprouted after awhile. It's the one tomato that I don't have a germination time for last year. The little yogurt cup ended up setting in a north window all summer. At some point a cat decided it wanted to look out the window and knocked the cup on the floor. It killed one plant and since the other was iffy I buried the stem sideways. After a period of time I ended up with two plants. A few weeks ago I transplanted the two into 16 oz cups that are now setting in a southern window. They've both grown a lot and one has buds on it. I figure sooner or later it will flower and I'll end up with 2 or 3 small tomatoes. They won't be big enough to eat but it should increase my seed supply from none to some and that is what I'm after. Assuming that the plants survive and I plant them in the garden I wouldn't be surprised that they produce fruit quickly and I also wouldn't be surprised that they produce ripe fruit no sooner than their off spring. Total time from planting to first ripe fruit. I'm guessing will be somewhere between 260 and 300 days.
I know this is an extreme example but it shows how much conditions matter.
As to when you plant you tomatoes out is going to depend on the conditions when it is time to plant them out. For me it's a mix of when does the soil get warm enough. Is it going to stop raining long enough so the holes I'm planting them in aren't little wells. Has someone offered to help me. That is the day some of them get planted. I've planted little ones and big ones and it doesn't seem to matter that much. General advice is six weeks before last frost I think it is, or is that 6 weeks before plant out time.