How does Heirloom know my farm’s climate for days to maturity?

From your farm’s location: Heirloom adjusts days to maturity to the climate where your farm is, day length included, so your harvest dates fit your land, not a seed company’s trial field. There is nothing to measure or enter yourself.

It uses your farm’s latitude and longitude, in your profile menu, Settings, General. They are filled in from your address when you set up your farm: check them if your address is a post office box or far from your fields. See How climate-adjusted days to maturity work.