"b. TESTING - Cylinders must be reinspected and
hydrostatically retested at least once every five years.
Testing must be performed in accordance with ' 180.205, DOT-
CFFC-13 and the latest edition of CGA pamphlet C-6.2"
Searching Google for the DOT-SP code (DOT-SP14387) turned that up from the hydrostatic testing renewal notice from PHMSA as the first link in PDF format.
That 5yrs starts from the last hydrostatic test date or from the "birth date," whichever is newer. In your case it's the "birth date," which was in January of 2k8. This means that in January of 2013, you need to have the tank rehydrostatically tested. You can do it before that date if you wish,
but I'd wait until the last possible moment to increase your time with the tank as much as possible.