Imam Ja'far al-Sadiq [a] quoted the Messenger of Allah [s] as saying:
"Whoever believes in Allah and the Last Day should keep their promise when they make it."
Our Holy Prophet Muhammad [s] teaches us the best morals and the most useful customs, and also how to fulfil a promise and be punctual.
Imagine that one day you have an appointment with your friend and have agreed to meet him half-an-hour before noon prayer in the mosque to recite the Holy Qur'an together. You arrived on time and waited until the prayer caller (Mu'adhin) called for the noon prayer. You performed the prayer but your friend still had not come. Certainly, you will blame your friend for not keeping his promise and making you wait. Hopefully your friend will feel responsible and apologise to you.
But how happy and glad you are if someone promises to do something for you and keeps his promise. Keeping a promise is among the morals of the true believer. And whoever does not fulfil their promise, no one will respect their word nor trust them.
It is obligatory for us to be honest in making a promise and keeping an appointment we have made. It is also obligatory upon us to be punctual about time and not to waste it. We should be exactly on time in school, at work, or at an agreed meeting place.
A Promise Fulfilled by our Prophet Muhammad [s]
Prophet Muhammad [s] promised a man that he would remain sitting in a certain place until the man returned. That man went away and the sun rose. Soon it became very hot but the Prophet never changed his place. One his companions said: "O Prophet of Allah! What difference does it make if you come and sit in the shade?" The Prophet replied: "I promised to meet him here and not to change a place till he comes, in order to fulfil my promise to him. If he does not come, then it will be he who is breaking the promise."
And Most Surely You Conform (Yourself) To Sublime Morality.
 Bihbodi, Sahih al Kafi, vol.1, p.117.