Allah (سبحانه وتعالى) says in the Quran:
يَا أَيُّهَا الَّذِينَ آمَنُواْ أَوْفُواْ بِالْعُقُودِ
"O you who believe, fulfil your promises....."
(Surah Al Mai'da: Verse 1)
Allah (سبحانه وتعالى) also praised Prophet Isma'il;
وَاذْكُرْ فِي الْكِتَابِ إِسْمَاعِيلَ إِنَّهُ كَانَ صَادِقَ الْوَعْدِ وَكَانَ رَسُولًا نَّبِيًّا
"Also mention in the Book (the story of) Isma’il: He was (strictly) true to what he promised, and he was an Apostle (and) a Prophet."
(Surah Maryam: Verse 54)
Keeping appointments is vital to life. Time is the most precious commodity. Once wasted, it can never be recovered. If you make an appointment, whether with a friend, Ulema or for business, you should do your utmost to keep this appointment. This somewhat simple adab is the right of the person you are visiting, as they have favoured you by sacrificing their valuable time for you. Lapse in timekeeping and failure to keep an appointmentnot only disrupts and causes inconvenience to the one you are visiting/meeting/arranged to see but also marrs your image and reputation. As your time keeping skills decline, peoples respect for you will also. Regardless of their importance, one should always endeavour to keep appointments;
وَأَوْفُواْ بِالْعَهْدِ إِنَّ الْعَهْدَ كَانَ مَسْؤُولاً
"....And keep your promises; the promise is a responsibility"
(Quran Surah Al-Isra: verse 34)
The Prophet (صلى الله عليه وسلم) once gave an appointment to one of his Companions. The Companion came 3 days later. The Prophet (صلى الله عليه وسلم) gently reprimanded him saying "You have caused me some trouble. I have been waiting for you for 3 days". The Companion probably had an excuse for this however he had no means by which to inform the Prophet (صلى الله عليه وسلم) of this in advance.
Today in an age of technology and reliable communication available everywhere, one would wonder why we have MST (Muslim Standard Time) or general widespread lateness amongst the Muslims. One should realise that we may infact be inconveniencing those who are to be visited. To seek an appointment is to seek permission. One should always endeavour to inform of the inability to make an appointment as that valuable time can be utilised elsewhere. No matter how unimportant a meeting may seem, it always merits an apology or prior notice of lateness, or cancellation.
An appointment is a commitment which shoould be kept properly or cancelled in advance.
In an age where the words "إن شاء الله" and "promise" are merely lipservice, one should be reminded of the clear prohibition of breaking promises and trust:
The Prophet (صلى الله عليه وسلم) said: "Three traits single out the hypocrite, even if he prays or fasts and claims to be Muslim: if he speaks he lies. If he makes a promise, he does not keep it. If he is entrusted, he betrays the trust."
(Bukhari & Muslim)
Imam Ghazali (رحمة الله عليه) explained that this hadith is applicable to those who promise while intending not to fulfil it, or those who, without excuse, decide later not to fulfil a promise. Those who promise but could not fulfil their promise due to a proper excuse are not hypocrites. One should be careful not to present a false excuse as Allah (سبحانه وتعالى) knows that which is in our hearts.
Never make a promise while intending not to keep it, for verily, actions are but by intentions. This is forbidden as it falls within lying and hypocrisy.