Did Allah Punish the People of Aad in One Day or More?
Brief description of the dubiety
When Allah, the Almighty, speaks about the destruction of the people of Aad in the Quran, there are three different amounts of time that are mentioned for that destruction. In verse 19 of Surah al-Qamar, it says that they were destroyed in a day; but in verse 16 of Surah Fussilat, it mentions that they were destroyed in a few days; while in verse 7 of Surah al-Haqqah, Allah clearly says that it took seven nights and eight days. Such a concept is impossible because this did not occur more than once, and its length can only be one amount of time.
Detailed description of the dubiety
Verses which mention that the punishment was one day
In verses 18 and 19 of Surah al-Qamar, it has been mentioned: “The people of Aad impugned their apostle. So how were My punishments and warnings? Indeed, We unleashed upon them an icy gale on an incessantly ill-fated day.” In this verse, the length of the punishment of the people of Aad is mentioned as one day.
Verse which mentions that the punishment was over a few days
It is stated in verse 16 of Surah Fussilat that: “So We unleashed upon them an icy gale during ill-fated days, in order that We might make them taste a humiliating punishment in the life of this world. Yet the punishment of the hereafter will be surely more disgraceful, and they will not be helped.” In this verse, Allah, the Almighty mentions the length of the punishment of the people of Aad as ‘ill-fated days.’
Verse which mentions that the punishment was over eight days and seven nights
Allah, the Almighty cites in verse 7 of Surah al-Haqqah that: “Which He clamped on them for seven gruelling nights and eight days, so that you could see the people there lying about prostrate as if they were hollow trunks of palm trees.” In this verse, Allah mentions an icy gale which was clamped upon them for a period of ‘seven nights and eight days.’
A summarized response
There are two possibilities regarding verse 19 of Surah al-Qamar on the basis of which, one can answer this dubiety.
The word «یوم» (yaum) in the Arabic language has two meanings: one meaning has the same English meaning as day, i.e. the length between sunrise and sunset; but the other meaning is a non-specific amount of time. In verse 19 of Surah al-Qamar, the word «یوم» refers to a non-specific amount of time, not the length of time between sunrise and sunset. Consequently, in this verse, there is no mention of a specific amount of time.
On the other hand, in verse 16 of Surah Fussilat, it is only mentioned that the punishment of the people of Ad took days, but it does not specify an amount. Only in verse 7 of Surah al-Haqqah did Allah mention the exact time length of the punishment. Therefore, these verses are not contradictory to one another in regards to the length of the punishment for the people of Ad.
It is also possible that the definition of the word «یوم» in verse 19 of Surah al-Qamar could refer to the beginning of the punishment, meaning that Allah sent an icy gale and storm towards them on one day. In this case, this verse only speaks about the beginning of the punishment and it does not refer to a specific amount of time. With this explanation as well, this verse would not contradict the other two verses.
A detailed response
«یوم» has two definitions
It is clear that there is no incompatibility between verse 16 of Surah Fussilat and verse 7 of Surah al-Haqqah because the first verse mentions very vaguely that the punishment of the people of Ad took a few days, but in the second verse, it has mentioned the length of the punishment accurately, and as a result there is no contradiction between these verses.
However, in regard to the 19th verse of Surah al-Qamar, one must pay attention to the definition of «یوم» in this verse. Several definitions have been mentioned for the word «یوم» including:
1. ‘The length of time between sunrise and sunset,’ which is the most well-known definition of «یوم», like in verse 59 of Surah Taha which states: “He said, ‘Your tryst shall be the Day of Adornment, and let the people be assembled in early forenoon.’” This verse mentions the agreed time between Prophet Musa and the Pharaoh’s magicians in order to clarify whether or not he was truthful. They chose the noon of the Day of Adornment. In this verse, «یوم» means the opposite of night.
2. ‘A non-specific amount of time.’  In this definition, «یوم» does not refer to the length of time between sunrise and sunset. For example, a poem states, «یوماه: یوم ندی و یوم طعنان» which means: “Two times, a good time and a difficult time!” Also, there is a saying, «تلک أیام الهرج» meaning: “Those tumultuous times;” and finally the famous saying of Amir al-Mumineen which is: «واعلم بأن الدهر یومان، یومٌ لک و یومٌ علیک» meaning: “Know that this life consists of two days - a day for you and a day against you.” 
From these examples, it is clear that what is meant from «یوم» does not refer to the opposite of night, but rather it means a non-specific amount of time. For this reason, it is possible that it could be referring to more than one day. This definition has also been used in the Quran. It is stated in verse 80 of Surah al-Nahl: “It is Allah who has made your homes as a place of rest for you, and He made for you homes out of the skins of the cattle which you find portable on the day of your shifting and on the day of your halt, and out of their wool, fur and hair [He has appointed] furniture and wares [enduring] for a while.” In this noble verse, it is clear that what is meant by «یوم» is not a single day of residence, but rather what it meant is a length of residence. In this verse, the definition of «یوم» is a non-specific amount of time. There are also other verses which use this definition of the word «یوم»; for example in verse 14 of Surah al-Balad it says: “Or feeding, during days of [general] starvation.” It is clear that this feeding means in times of starvation, not just the period of time between sunrise and sunset.
In verse 19 of Surah al-Qamar, the second meaning is given, in other words a non-specific amount of time, not a seventh of a week, or the time between sunrise and sunset. Just like sometimes we may say: “One day I caught a cold.” In this context, day refers to a non-specific time.  
Someone who has a lot of money witnesses that his child spends his money carelessly. He advises his son to put a limit on how much he spends, and eventually he says to him: “Now you have a lot of money, but think about the day that you will be poor.” If at another time he says to him: “The days of your poverty will be difficult,” there is no contradiction between the two statements because in the first sentence, he did not refer to a specific day when he used the word ‘day,’ but rather he was referring to a time when he would become poor.
«یوم» refers to the beginning of the punishment
As mentioned in the first answer, there is no contradiction between verse 16 of Surah Fussilat and verse 17 of Surah al-Haqqah, but there is another possibility about verse 19 of Surah al-Qamar which is that the meaning of sending the punishment is referring to the beginning of the punishment. It is stated in this verse, “Indeed We unleashed upon them an icy gale on an incessantly ill-fated day.” According to this verse, a punishment of an icy gale was sent to them one day, meaning that the fate of these people was such that Allah eventually unleashed a terrifying icy gale upon them. As well, this verse does not speak about the length of the punishment, but rather, it talks about the fate of the people of Ad upon whom a punishment was unleashed eventually.
Imagine someone says the following three verses in regards to the USA war on Vietnam:
1. The United States’ Military attacked Vietnam one day.
2. The United States’ Military fought in Vietnam for years.
3. The United States’ Military fought for almost twenty years in Vietnam.
There is no contradiction whatsoever between these three sentences - the first sentence only refers to when the war began, and it does not mean day which is the opposite of night. The second and third sentences which talk about the length of the war are not contradictory either, because in one instance, the duration has been mentioned vaguely, while the other one has stated the length accurately.
Similar is the duration of the punishment of the people of Ad, and in the verses mentioned above, there is no contradiction in them at all.
- Mufradat Alfaz al-Quran, Page 894
- Nahj al-Balaghah, letter 72
- Tafseer al-Mizan, Volume 19, Page 70
- Mafatih al-Ghayb, Volume 29, Page 304