This is an issue which confuses some people, and some of them think that Allah created the heavens and the earth in eight days as Allah says in Soorah Fussilat (interpretation of the meaning):

“Say (O Muhammad): Do you verily disbelieve in Him Who created the earth in two Days?And you set up rivals (in worship) with Him?That is the Lord of the ‘Aalameen (mankind, jinn and all that exists).

He placed therein (ie the earth) firm mountains from above it, and He blessed it, and measured therein its sustenance (for its dwellers) in four Days equal (ie all these four 'days' were equal in the length of time) forall those who ask (about its creation).

Then He rose over (Istawa) towards the heaven when it was smoke, and said to it and to the earth: ‘Come both of you willingly or unwillingly.’ They both said: ‘We come willingly.’

Then He completed and finished from their creation (as) seven heavens in two Days and He made in each heaven its affair.And We adorned the nearest (lowest) heaven with lamps (stars) to be an adornment as well as to guard (from the devils by using them as missiles against the devils).Such is the Decree of Him, the All-Mighty, the All-Knower”

[Fussilat 41:9-12]

because this seems to contradict the other verse which says that He created them in six days.

This is a misunderstanding, and the answer to it is as follows:

There is no contradiction between the time period mentioned in these verses and the other verse which says that it was six days. 

In these verses – from Soorah Fussilat – we see that Allah is telling us that He “created the earth in two Days”.  

Then He “placed therein (i.e. the earth) firm mountains from above it, and He blessed it, and measured therein its sustenance (for its dwellers)” in four days equal– i.e., in two days that were added to the two days in which He created the earth, so the total is four days.  It does not say that the creation of the mountains and the measuring of the sustenance took four days. 

Perhaps the confusion which is mentioned in the question stems from this, i.e., from thinking that the four days are added to the two days in which the earth was created, equaling six, and then adding the two days in which the heavens were created (“Then He completed and finished from their creation (as) seven heavens in two Days”) – making a total of eight days, not six days. But this confusion can be dispelled by dealing with this mistaken notion.  So the earth was created in two days, and the mountains were created and the sustenance measured in two more days which makes a total of four, i.e., this took the other two days. Then the creation of the seven heavens took two days. So the total is six days of the Days of Allah, may He be glorified and exalted. 

Tafseer al-Baghawi, 7/165 

Al-Zajjaaj said: “in four days” means two days added to the previous two days. 

These verses – from Soorah Fussilat – confirm the other verse, which says that the creation of the heavens and the earth was completed in six days. So there is no contradiction concerning the period in which Allah created the heavens and the earth. There cannot be any such contradictions in the Qur’an..