Two rules, two results
Under digit-sum reduction, 28 becomes 2+8=10. Under repeated subtraction, 28 becomes 28−22=6. For 25, digit addition gives 7 and subtraction gives 3. Values already from 1 to 22 remain unchanged in both conventions.
Neither convention is the same as reducing everything to a single digit. In our implementation, 19 and 22 retain their full Arcana identity. The methodology page names the chosen rule and shows examples.
Compare 28 November 1987
Using digit sums, the reduced day is 10, month is 11, reduced year is 7, bottom is 10, and center is 11. With repeated subtraction, the day is 6, month 11, year 3, bottom 20, and center 18.
The difference propagates into diagonal and inner positions. It is therefore misleading to compare only the center and conclude that one implementation has an arithmetic bug. First compare the documented formula.
Reduction timing also matters
After completing a node, reduce it before using it in another personal formula. Carrying an unreduced total forward can change downstream values. In compatibility calculations, complete both individual charts before pairing their corresponding positions.
This distinction deserves tests. A calculator can appear correct on easy dates while failing when an intermediate value exceeds 22. Worked fixtures containing 22 and multiple reductions expose those errors.
Make a useful comparison
Write the date, method, position label, raw sum, and reduced result. Compare those details across calculators. If conventions differ, the discrepancy may be expected. If they match and the arithmetic differs, you have a specific issue to investigate.
Do not interpret a disagreement as evidence that one chart reveals a more accurate future. The numerical construction and the symbolic interpretation are separate questions.
A question to carry with you
What part of this reflection can I check against my own experience?
References & context
References for the calculation method and Arcana tradition.