The visa stamp in your passport is from the US embassy or consulate. It lets you travel to the US and ask to come in. Its expiration date is only about entering.
The I-94 is from CBP at the airport. It says how long you may stay this time.
So a visa can expire while you are inside the US, and you can still be here legally. Example: your F-1 visa expires in June 2027, but your program ends in May 2029. You may stay and study until the program ends. You only need a new visa for the next time you enter the US after June 2027.
The opposite is also true. A 10-year B visa does not mean you may stay 10 years. Each visit is limited by the I-94 date.
D/S means "duration of status". Most F-1, J-1 and their F-2 and J-2 families get D/S instead of a date. You may stay as long as you follow the rules of your program:
F-1: as long as your I-20 is valid and you study full time, plus a 60-day grace period after you finish.
J-1: as long as your DS-2019 is valid, plus a 30-day grace period.
In July 2026 the government published a rule to replace D/S with fixed dates for F and J students. A federal court blocked it on September 14, 2026, so D/S still applies while the court case and the government's appeal continue. This can change. Ask your school's international office (your DSO or RO) if you hear news.
Which date do you follow?
Follow the Admit Until Date on your I-94, or the program dates for D/S. Not the visa stamp.
If an employer extension or change of status was approved by USCIS while you were in the US, the new I-94 is at the bottom of the approval notice (I-797A). That newer I-94 is the one that counts until you travel again.
Thinking an expired visa stamp means you must leave. It does not.
Thinking a long visa means a long stay. It does not.
Forgetting that the passport must stay valid. Renew it before it expires. See Renewing your passport from the US.
Staying even one day past the I-94 date. Overstaying cancels the visa and can block future visas.