Temporary employees can be admitted into the United States 10 days prior to the validity period granted on their I-797 Approval Notice, and, in most cases, they will be allowed to remain in the United States for 10 days following the validity period granted on their I-797 Approval Notice. The I-94, issued by CBP, will reflect these two 10-day periods if they are granted upon admission into the United States. These two 10-day periods operate solely as a grace period. To elaborate, these two 10-day periods help facilitate travel, help to maximize benefits of nonimmigrant visas, etc. However, these two 10-day periods do not grant work authorization, and temporary employees cannot work during this time. The source of this information can be found in title 8 of the Code of Federal Regulations. The provision, current as of 12/16/2016, is below for reference:
8 C.F.R. 214.2(h)(13)(i)(A)
§ Sec. 214.2 Special requirements for admission, extension, and maintenance of status.
(A) A beneficiary shall be admitted to the United States for the validity period of the petition, plus a period of up to 10 days before the validity period begins and 10 days after the validity period ends. The beneficiary may not work except during the validity period of the petition.