If there is a 15 day period in a contract, when does that begin - date of contract signing, or when signed contract received?

Generally speaking, a contract is "fully executed" when both parties sign it, as long as there are no changes that need to be agreed to by the other party. Certainly, all parties need to receive a copy of the fully executed contract, but the actual execution date is the date it is signed by all parties. The exception to this would be where a specific date or event is listed within the contract itself, stating that the contract would not be considered "fully executed" until all parties are in possession of their copies. This would be unusual, but legal.