The basics
How a B-2 visitor extension works
A B-1/B-2 extension lets you stay longer for the same visitor purpose (tourism, visiting family, medical treatment). USCIS guidance is to file the I-539 before your current status expires — they generally recommend applying at least 45 days before the Admit Until date when you can, and you must file before it passes. You also need to show you still maintain status, can support yourself financially, and intend to return home. This app builds the whole package; you just print, sign, pay and mail.
What if…
What if my Admit Until date is very close?
File as soon as you can — what counts is that USCIS receives it before the date passes. Use the chat to move quickly; the assistant shows how many days you have. If the date has already passed, that is a situation to review with a licensed immigration attorney.
Official sources
This is general information, not legal advice.