What you have to bring
The seed-starting countdown counts back from an outdoor date by the weeks you enter. Those weeks are the “start indoors” line on your packet. If the packet does not give a number, there is no count to make.
The outdoor date is one of two choices on the page. The median last 32°F date is the 50% spring probability at the station for your ZIP. The other choice is the first day of a cited outdoor window, available when that window has a start date. Without a cited window, the countdown uses the median.
How the date is produced
Enter a whole number of weeks from 1 to 20. The countdown multiplies that count by seven and subtracts the days from the anchor. If the anchor’s month and day has already passed this year, the tool uses the next occurrence that has not passed, then counts back from that future date. The sentence names both the sow date and the transplant date, and it names which anchor was used, so you can see the inputs.
If the sow date is already past, the packet’s indoor start for the coming transplant date has already begun. The tool says that. It does not shift the transplant date later to make the sow date land on today. Changing the plan is your decision. Changing the anchor, or typing a different week count from a different packet, is how you ask a different question.
What the countdown leaves out
Indoor weeks are not a frost forecast, and they are not the outdoor planting window. A packet can say six weeks before transplant while the extension guide says to set plants out one to three weeks after the frost-free date. Those are two different instructions. Use the countdown for the indoor start. Use the planting calendar for the outdoor window. The seed-starting guide keeps that split, with Texas A&M’s outdoor timing as the worked example.
The countdown also ignores germination temperature, light, and how long your seedlings actually sit in a tray. A cold room can add days the packet did not mention. The NOAA date does not know about your windowsill. Treat the sow date as packet arithmetic against a named station date, then check the plants you have, not the date alone.
The median date, and the reason a guide’s weeks use the 50% column when the guide names no probability, are explained on the methodology page.