XTide reads this data from harmonics files that you must download along with the distribution. Ultimately, XTide's predictions can only be as good as the available harmonics data. It is up to you to verify that the predictions for your locale match up acceptably well with the officially sanctioned ones, and to let me know if they do not. Deviations of 1 minute from official predictions are typical for U.S. locations having the latest data. Deviations of 20 minutes are typical for non-U.S. locations or U.S. locations that are using obsolete data.
Much longer deviations indicate a problem.