Transplanting seedlings into your garden / final container, though it's a rough estimate at best.

The DTH (Days to Harvest) or DTM (Days to Maturity) numbers you see on seed packages are that, with the caveat that conditions (temp, soil fertility, etc.) can run roughshod over those numbers. The ending day is the first mature fruit. But the starting day depends on the plant. If you've got plants that are usually started from seed in the soil, it's when the seed gets planted. Period. Quite often, for things that are bred as seedlings before being transplanted out, it's from the transplant date. That applies to tomatoes. That's a slightly screwy idea, because some people grow "seedlings" for months before planting out. But I guess you can't really call them "seedlings" when they're a foot tall! Now, the speed at which seedlings grow depends strongly on temperature. So presumably, what they mean is, from the time they are transplantable - namely, a few inches tall. Again, depending on temperature, for tomatoes, that's 3-4 weeks after being seeded.

I should say that those numbers aren't best used for planning harvest dates, but are more useful for comparing speeds of different kinds of plants. That is, if you have an early frost, small DTM plants are advisable.

It means it's not quite an early tomato, but pretty close. But yeah, that's the days from the transplant. If that was from seed, that would be super early (although there are tomatoes like that).