How do you tell how old hard hats are?

Hardhats usually have a circle on the inside that has the numbers 1 through 12 and an arrow pointing at one of the numbers, this indicates the month it was manufactured. Secondly the year is inside the same circle expressed in the last two numbers. So if you find a circle with the number 05 in it and the arrow pointing at number 8, it means it was manufactured in august 2005.