All of the hydro dates on my tanks follow this format:
BIG NUMBER four little numbers or letters in two rows BIG NUMBER
The first big number is the month, I don't know what the four little ones are, the last big number is the year. This gives you the month and year of the last test. There may be several usually about 5-7 years apart. Find the most current one. If this doesnt make sense I will take a picture later.