What the back-calculator does
The fall tool counts backward from a first-32°F probability date at the station matched to your ZIP. You type the days to maturity from your seed packet, and you may type extra days you want as a cushion before that frost date. The default cushion is zero. The result is the sow-by date: frost date, minus the days you typed, minus the extra days.
The count uses your packet and the NOAA date. When the planting calendar prints the guide’s own fall window, that window is the extension date, shown beside the count. The calculator also runs in states that have a station and no cited crop guide.
Which frost date to pick
The menu offers the 10%, 50%, and 90% first-32°F dates. On a first-freeze row, 10% is the earlier date and 90% is the later date. The 50% date is the median. Counting back from the 10% date moves the sow-by date earlier, which leaves more of the season after sowing and treats the early tail of the record as the deadline. Counting back from the 90% date moves the sow-by date later. Neither choice is “the” fall planting day. They are three readings of the same station.
If that month and day has already passed this calendar year, the tool places the frost date on the next occurrence that has not passed, then counts back from there. Days to maturity must be a number from 1 to 400. Extra days must be from 0 to 120. Anything outside those bounds is rejected instead of producing a date.
What to type, and what not to trust
Type the days printed on the packet you are holding. Days-to-harvest notes in an extension guide are a different source, and they describe a range for a crop, not your variety. If the packet says 55 days and the guide says 50 to 70, the tool should get 55, or whatever number you choose to type. It will not look the number up for you.
The sow-by date assumes the plant needs that many days before the frost date you picked, and that the frost date is a fair deadline for that crop. Some crops tolerate a light frost. Some do not. The tool does not know which, because the NOAA file is a temperature probability, not a crop list. Compare the result with the cited fall window when one is printed, and with the packet’s own season. The fall timing guide shows how one extension chart lines up with a median first-32°F date, and where the two should not be blended into a single instruction.