How else would we translate and there was evening and morning, ONE DAY. ~wy or the transliterated word YOWM is day and it occurs 137 times in Genesis. If we look at the other usage what other meaning could be implied?
All the "yom" entries in Gen 1 mean an indefinite period of time. Gen 2:4 refers back to them and says "in the yom" God created the universe. God could not have created the universe in both one 24 hr day and 6 24 hr days. There is no alternative but to translate yom in this context as "indefinite period of time".
It works much better that way with reality too. In fact, the "let there be light" almost makes the rest of Gen 1 unnecessary, once you understand Einstein's formula for coversion of mass and energy.