The Bible offers all the clues we need to figure it out.
Back in biblical times, in order to be accepted as a priest there was an age requirement. The Torah was very clear in Numbers 4:3 that a man must be at least 30 years old to hold such an office. This would mean that when Jesus started his ministry, after being baptized by his cousin John, he must have been at least 30 years old. Luke 3:23 tells us as much, saying, “Jesus himself began at about thirty years of age.”
But this does not follow with what we know of Jesus’ timeline. If we were to go by the generally accepted notion that Jesus was born in Autumn of the year 3 BC and died after Passover in 30 AD, then he would have started his ministry at 26 AD. This could work, if there were 30 years between 3 BC and 26 AD, but the bible counts years a bit differently than we do.
Historical journalist David Price tackled these biblical quandaries by taking a look at how the Bible counted years. In the Bible, years were counted by the reigns of kings, but they always record the length of rules by full years. There was a specific day on the year, known as the Feast of Trumpets, which marked the end of a regnal year, and whoever ruled at that point got credit for the year. For example, if a king assumed the throne just a month before the Feast of Trumpets, then he was credited with reigning for the entire year.
Price works the math out for us with the help of Dr. Earnest L. Martin:
According to the calculation made by Dr Ernest L. Martin in his book, The Star that Astonished the World, Jesus was born at the beginning of the first of Tishri at the start of the Feast of Trumpets, 3 BCE. So we can calculate: 3 BCE to 2BCE is one year. 3 BCE to 1 BCE is two years. Then we have to remember that there was no year zero. The next year is 1 CE. 3 BCE to 1 CE is 3 years. 3 BCE to 6 CE is 8 years. 3 BCE to 26 CE is 28 years!
This still leaves us with a two-year discrepancy. Now we must remember that Jesus was born on the Feast of Trumpets (very appropriate for the arrival of a new king). If the Trumpets sounded at mid-day and Jesus was born the night before that could mean that Jesus’ first year was only a few hours long, since Hebrew days begin and end at sunset. The second calculation brings us closer.
So we must count the time before the trumpets of 3 BCE as the first year of Jesus’s life. The second year started with the Trumpet and continued to the next first of Tishri in 2 BCE. So the calculation should read as following: Birth to the time of Trumpets 3 BCE year one 3 BCE to 2 BCE 2 years. 3 BCE to 1 BCE is three years. 3 BCE to 26 CE is 29 years.
With only one year off, Price looked further:
Jesus was beginning his thirtieth year. Come the 1st of Tishri, this would all be recognized as his year. Under Hebrew law he was entitled to count this year. As Jesus was alive on this next feast of Trumpets, we can say with Luke Jesus was beginning to be thirty years and fully qualified to be priest.
This is just one interesting excerpt from an all together fascinating article which can be read here.