What is the ruling on someone who swore by that which is unlawful to accomplish a specific task at a specific time, then returned to attempt to accomplish it after failing to do so within the specified time?

It is not permissible to swear by other than Allah. Whoever swears by that which is unlawful concerning someone other than his wife is liable to an expiation for breaking an oath, based on the Almighty's saying: "O Prophet, why do you prohibit [yourself from] what Allah has made lawful for you, seeking to please your wives? And Allah is Forgiving and Merciful. Allah has already ordained for you [Muslims] the dissolution of your oaths."