Days are best described as periods of time or stages. We are told that a Day in the sight of God is like a thousand years of our reckoning:

A Day in the sight of thy Lord is like a thousand years of your reckoning.
[Qur'an 22:47]

In another verse, the comparison is with 50,000 years:

The angels and the Spirit ascend unto Him in a Day the measure whereof is as fifty thousand years.
[Qur'an 70:4]