The accuracy of any pH meter can only be assured when it is calibrated with standardized buffered solutions. Usually this is done at 2 points, pH 7.01 and pH 4.01 or 10.01 depending on the range in which you are working.
For cheesemaking the choice is pH 7.01 to 4.01 which includes the range in which milk moves with the bacteria and lactic acid production.
Hanna's buffer solutions are prepared according to precise formulas and are standardized with a pH meter calibrated according to the NIST primary standard buffer solutions.
Expiration dates are current from manufacturer.