DTM is determined by the average (not median) number of days it takes the particular crop to mature in optimal conditions in the zone where the test crops are grown by the seed producers for a particular seed distributor. This is why you see different DTM on seed packets bought from different distributors.

Since so many of the seed distributors in the US now obtain many of their seeds from overseas the DTM they publish are not really applicable to the US gardeners.

That said, DTM for vegetables normally grown from transplants is from the day they are transplanted to their final growing place. DTM is not size related and means nothing if they are direct seeded.

DTM for vegetables normally direct seeded is from the day the seeds are planted in what will be their growing place and means nothing for those same seeds started indoors and later transplanted.

DTM for all vegetables is only determined by what is the standard practice for sowing them. We can't have it both ways. No big deal since it means so little anyway.

But what does days to maturity mean for those vegetables that are done both ways (lettuce for one) and is there a standard list of vegetables that are transplanted/direct sown?

There are very few, if any, vegetables that have a standard practice of "doing it both ways" so no published DTM for them. Lettuce has a direct seeded standard practice attached to it. So the DTM one finds for them will always be based on that standard.

You can 'estimate' a DTM by adding the number of days it took to germinate it inside to the DTM on the package and add approx. 2 weeks since that is the amount of set-back transplanting causes. But doing so makes the end info even less accurate.

Now factor in the fact that few gardeners have or provide "optimal conditions" and that the BIG variable - weather -varies drastically from zone to zone and even within zones and you begin to see the problem.

Yes there are many lists but how accurate can they be since they just use standard practice. And even they are zone specific as what is 60 days for lettuce in southern CA is more likely 70-75 days for the same lettuce in say Idaho.

So what is the solution? Detailed record keeping for a couple of years in your own garden. Note the day you direct seed some lettuce inside to be transplanted and the same lettuce outside, the date it germinates both places, the date it matures from both plantings. Then you have your DTM for that lettuce variety IF the weather cooperates from year to year.

Hope this helps even though it likely isn't what you want to hear.

Dave