How mature did you think you were at age 16?
And at age 21 you realized NOW you were really mature?
How mature does a 30 year old think a 21 year old is?
Likewise, is a beet mature when you can harvest the beet, or when you can start eating the greens?
Is a tomato mature when it produces that first bud or when you can pick a ripe fruit?
Some questions I can't answer, but here's what I can share:
1. Days to Maturity, or DTM, is calculated for plants that are started indoors from the time you transplant them outdoors. Take Broccoli as an example. If the seed packet instructions indicate to start seeds indoors 6 weeks before the last spring frost, and the DTM is 55 days, it means that 55 days from transplanting outside is the approximate time you should start seeing a head on your broccoli plant.
2. DTM for direct seeding is from the time the seedling emerges from the soil. This can happen in just a few days for some veggies like peas and beans, or can take 3 weeks and more for others. So in the case of broccoli, if the seed packet does not give you the DTM for direct seeding, add on a few weeks. Not the entire 6 weeks from starting indoors, though; only about 3 weeks. Here's why:
3. When you transplant any veggie, you lose some development time. Again looking at broccoli, when you put it in the garden you set it back a few weeks. If you have ever transplanted a plant you probably have noticed that they kind of sit there for a while, before they take off. They may even look like they are suffering. This is known as Transplant Shock.
4. The same veggie will have different DTM for different varieties. If you look through any seed catalog you may find upwards of 3 weeks different DTM for the same veggie. This is important if you are timing your veggie in regards to heat or frost, if you are succession planting, and if your growing season is limited.
5. The weather changes everything. All the charts and lists go out the window if there is more or less rainfall, unusual highs or lows, etc. DTM numbers are based on how the plants perform on the average, and often under greenhouse conditions. How soon a plant will mature in your garden will be different than what will happen in ours.
So use the DTM's as a guideline, and keep notes if you want to get more accurate numbers for your own garden.
Isn't that part of the fun of gardening after all?