Does climate-adjusted DTM work in a greenhouse or tunnel?

They are built for crops in the open field, and under cover your own numbers are the better guide. A greenhouse or tunnel is warmer than the weather outside, so crops there usually mature faster than the prediction.

For a planting in a greenhouse, Heirloom shows a message that the prediction may not be accurate. Tunnel plantings show no message, so keep in mind that the prediction is made for the open field. Under cover, leave Use climate-adjusted DTM off and enter the days to maturity you see in your own beds. See Understand the "Outside optimal period" warning.