In the past, when a person entered the U.S. as a visitor under the Visa Waiver Program ("VWP"), a little green card called an I-94W was issued to the traveler. The I-94W would show when and where the person entered the U.S. It would also show when that person's visitor status would expire (90 days after entry).
Recently, however, the practice of issuing I-94W cards was eliminated. Now, the Immigration Inspector at the port of entry will stamp just the passport and only write either "WB" or "WT" (Waiver Business or Waiver Tourist). There is no policy or requirement to write at the bottom of the stamp the expiration date of the person's visitor status. The Immigration Inspector is also not required to advise the visitor when his or her status expires.
This could be very problematical and could cause people to overstay. Once a person overstays under the VWP, that person can never again legally enter the U.S. under the VWP. That person must always have a valid visa in order to enter the U.S.