You are right that God is outside of time and to Him a day not the same as a day is to us (Ps. 90:4, 2 Peter 3:8), but one of the first things God did in the creation story was give us a reference.
Genesis 1:3-6, And God said "Let there be light," and there was light. God saw that the light was good, and he separated the light from the darkness. God called the light "day," and the darkness he called "night." And there was evening and there was morning, the first day.
While there are many important parts of these verses, for time determination the last sentence is operative. The evening and the morning, a 24 hour period, was determined to be the first day. Most people say that all God created on the first day was light, but that is not true. The other thing that God created on the first day was time.
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