Err... what Old Testament are you using? Yom has many different uses in the Old Testament.
For example, in 67 verses in the Old Testament, the word Yom is translated into the English word "time." For instance, in Genesis 4:3, it says "And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the Lord." In this instance, Yom refers to a growing season, probably several months. Again, in Deuteronomy 10:10, it refers to a "time" equal to forty days. In I Kings 11:42, it says "And the time that Solomon reigned in Jerusalem over all Israel was forty years." In this case, Yom translated as the word "time" is equivalent to a 40 year period. In Isaiah 30:8, it says "Now go, write it before them in a table, and note it in a book, that it may be for the time to come for ever and ever." In this case, Yom is equal to "forever." How long is forever? An infinite number of years...billions upon billions upon billons of years. If Yom can equal trillions of years here, then why not billions of years in Genesis?
Four times in the Old Testament Yom is translated "year." In I Kings 1:1, "David was old and stricken in years..." In 2 Chronicles 21:19, "after the end of two years" and in the very next verse "Thirty and two years old." Finally, in Amos 4:4, "...and your tithes after three years." In each case, Yom represents years, not days.
Eight times in the Old Testament Yom is translated "age." These range from sentences like "stricken in age," meaning old age (Genesis 18:11 and 24:1; Joshua 23:1 and 23:2), and other times it says "old age" (Genesis 21:2, Genesis 21:7). Genesis 47:28 refers to "the whole age of Jacob," therefore yom here refers to an entire lifetime. In Zechariah 8:4, it says old men and women will sit in the streets of Jerusalem, "each with cane in hand because of his age."
One time Yom is translated "ago." 1 Samuel 9:20 says "As for the donkeys you lost three days ago, ..."
Four times yom is translated as "always," in Deuteronomy 5:29, 6:24, 14:23, and in 2 Chronicles 18:7. Always here can be interpreted as a lifetime...for instance, we are to keep the commandments of the Lord always (Deut. 5:29).
Three times yom is translated "season." In Genesis 40:4, "...and they continued a season in ward." Again, in Joshua 24:7, "dwelt in the wilderness a long season," and in 2 Chronicles 15:3, "...a long season Israel hath been...". In each case yom represents a multi-month period.
Then I guess those are the translations!
Those who insist on yom meaning a literal 24-hour day have to deal with the fact that every scientific observation contradicts their theology.
Those who interpret yom to mean an indeterminate period of time -- which is completely compatible with other uses of yom in the Bible -- find that there is no contradiction between science and the Bible.
...or, those translations are wrong!