The Hebrew word for “day” is the word “Yom.” Young earth creationists have always argued that the word used for the days of creation can only mean a 24-hour day. In this article, we will examine the uses of Yom in the Old Testament, and show that it can mean a wide variety of time periods.
 
First, one must understand that the Hebrew language is not nearly is diverse as our English language. Whereas we have millions of words, the Hebrew source for the Old Testament only consists of slightly less than 8,700 words...and many of these could be considered duplicates with only slight differences. Thus, words which contain multiple meanings are common. Such is the case with the word Yom.
 
Hebrew Dictionaries
 
Let’s start with the possible meanings of Yom;

The Theological Wordbook of the Old Testament (1980, Moody Press)

"It can denote: 1. the period of light (as contrasted with the period of darkness), 2. the period of twenty-four hours, 3. a general vague "time," 4. a point of time, 5. a year (in the plural; I Sam 27:7; Ex 13:10, etc.).

"Hebrew authorities attest to the fact that the word Yom is used for anywhere from 12 hours up to a year, and even a vague "time period" of unspecified length. 

Other Uses of Yom

Day is not the only translation for the word Yom. Here are some other uses.

Time

It is interesting to note that 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. In Deuteronomy 10:10, it refers to forty days. In I Kings 11:42, it refers to 40 years.

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." 

Year

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.

Other Meanings for Yom

Age meaning old age (Genesis 18:11 and 24:1; Joshua 23:1 and 23:2)
Ago 1 Samuel 9:20
Always (Deuteronomy 5:29, 6:24, 14:23, 2 Chronicles 18:7)
Season (Genesis 40:4, Joshua 24:7, 2 Chronicles 15:3)
Chronicles  (27 times)
Continually  (11 times). 

Ever (Deuteronomy 19:9; eighteen other times.  The old testament uses "for ever" instead of the word forever. In sixteen cases of use of the word ever, for is placed before it, indicating a infinite period of time (Last word of Psalm 23:6) 
Evermore (Deuteronomy 28:29)

Word Usage in the Old Testament

As you can see, Yom is used in a wide variety of situations related to the concept of time. Yom is not just for days...it is for time in general. How it is translated depends on the context of its use with other words.

Yom in the Creation Account

Even within the creation account, Yom is used to represent four different time periods. 
Genesis 1:5 "And God called the light Day, and the darkness He called Night." Here, Moses uses Yom to indicate a 12-hour period

Genesis 1:14 "And God said, "Let there be lights in the firmament of the heaven to divide the day from the night, and let them be for signs, and for seasons, and for days, and years." Here, Moses uses Yom to indicate 24-hour days

Genesis 2:4 "...in the day that the Lord God made the earth and the heavens." Here, Moses uses Yom to indicate the entire creative week.

The fourth usage of Yom in the creation account is in the summary for each of the six creation days, "and there was morning and evening the first day". Yom is used to represent a finite, long period of time, usually either millions or billions of years. To show support for this, consider the uses of Yom by Moses.

Moses Other Uses of Yom

Moses, the author of the first five books of the Bible, and of Psalm 90, used Yom in many different ways.

Genesis 4:3 "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. 
Genesis 43:9 "...then let me bear the blame for ever." 

Here, Moses uses Yom to represent eternity
Genesis 44:32 "...then I shall bear the blame to my father for ever." Again, Moses uses Yom to represent eternity

Deuteronomy 4:40 "...that thou mayest prolong thy days upon the earth, which the Lord thy God giveth the, for ever." Here Yom represents a physical lifetime

Deuteronomy 10:10, "Now I stayed on the mountain forty days and nights, as I did the first time,..." 

Here, Yom is a "time" equal to forty days.

Deuteronomy 18:5 "...to stand to minister in the name of the Lord, him and his sons for ever." 

Again, Yom is translated as eternity

Deuteronomy 19:9 "...to love the Lord thy God, and to walk ever in His ways..." Here, Yom represents a lifetime. As long as we live we are to walk in his ways

As you can see, Moses used the word Yom to represent 12-hours, 24 hours, the creative week, forty days, several months, a lifetime, and eternity. 

Common Young Earth Arguments

To get around the obvious conclusion that Yom in Genesis 1 can mean millions of years, young earth theorists have come up with several arguments, none of which is supported by common Hebrew grammatical rules according to Hebrew experts. These rules were created by Hebrew language experts who are young earth creationists, thus their viewpoint is obviously biased. They have a specific agenda they are trying to prove, and thus cannot be objective.

Ordinals/Cardinals

Whenever Yom is used with an ordinal or cardinal number (1st, 2nd, 1,2, etc) that it always represents a 24 hour day. However, this is not true. In Zechariah 14:7-9, the "one day" refers to a period of time when the Lord shall be king over the earth. In other places, some say that Isaiah and Hosea have numbers with the word day which are figurative (External Link). 1 Hebrew scholars do not recognize this fabricated rule.2 

What is important in the Hebrew language is the intentions of the author. We should not create rules that support our own agendas, but should strive to understand the author's intended meaning outside of rules.

Evening/Morning Construction

In Genesis 1 Moses says "and there was evening and morning the xx day". Does the use of evening and morning indicate a sunrise and sunset for each creative day? First, let's look at what evening and morning are not. They are not actual evening and mornings, as this requires a sunrise and sunset. According to young earth theory, the Sun was not created until Day Four, thus there could be no sunrise or sunset for the first three days of creation. However, God uses the terms evening and morning for those first three days. 

Therefore, they cannot be actual evenings and mornings. 

We are left with only one option. The words for Evening and Morning can only represent the beginning and ending of the creative period, and not actual sunrise and sunsets. Scripture itself sets this pattern for us. Morning and evening are used figuratively in Psalm 30:5, Psalm 49:14,15, Psalm 90:6. Thus, the evening and morning of creation can mean the start and end of the creative process that is attributed to that creation period.

Literal/Figurative Argument

This argument says that you cannot use a word figuratively until after you have used it literally. Is this rule valid? I see no reason to suppose that it is. You have to be careful with young earth claims about biblical interpretation methods. Again, they will invent rules that support their cause, when there is no basis for their rule in Hebrew.

In this case, it makes no difference which order the word Yom appears in, i.e. literal before figurative or vice versa. Yes, these are the first words of the Bible, but they are not the first words of mankind. All the time from Adam to Moses, men were speaking in their own languages, thus the literal interpretation via spoken language would already have been established. There was no need to suppose a literal/figurative structure.

If God's Creation Was Billions of Years Old...

If God's creation was billions of years old, how would He have written the creation account in Genesis? One thing is certain...God is good at telling us exactly what we need to know.

When God refers to a large number, He uses picture stories, such as Abraham's descendants being as numerous as the sand. Why does He do this? If God had said, "You will have millions of descendants," Abraham would have asked, "What is a million?" 

When considering the creation, if we broke it down into days, that would be 5,000,500,000,000 days, or roughly 13.7 billion years. Do we need an account for each day of creation...of course not. 

God in His infinite wisdom, saw fit to tell us the creation story by breaking it down into creative segments, each of which was attributed to a specific creative act or acts. We need to give the early Hebrews of Genesis a break...they didn't have calculators like we do!

One must also consider that time with God has no meaning. To Him, 10 billion years is like a day. Thus, it is no problem for God to put billions of years into one of His days. Dr. Hugh Ross puts it best in his determination that the frame of reference for creation is the surface of the earth. Genesis 1:2 puts the witness of creation on the surface. But who is witnessing these events? It is God himself. During the first 5.99 days of creation, God is the only one present. Thus, human time does not matter...no humans were there to witness the passage of time. What matters is how God sees time! Thus, a billion year day is only a passing moment in God's eyes.

The creation account is written in such a manner for all people to understand it. The issue is not how long creation took...the issue is that God did it, and that's all that matters in the end.

Conclusion

With such a wide usage of the word Yom for many different time periods, it cannot be claimed that Yom in the Old Testament only represents a 24-hour period. During the creation account alone, Yom represents four different time periods. Rules of Hebrew, created by young earth Hebrew scholars, are invalid. Because of their biased position, they are trying to prove their own agenda.

Since humans did not witness creation, our own concept of a 24-hour day does not apply..