My understanding is that your visa stamp is for the purpose of leaving Nepal and to enter the USA only. After entering from port of entry, your stay is governed by I-94 and you are authorized to stay as long as your I-94 permits regardless of your visa stamp expiration date. In this case, i think you can stay longer if you apply for extension of authorized stay by filling I-539 to the USCIS with genuine reason and they approve your petition. In that case, they will issue a new I-94 and it will show how long you are authorized to stay.