The description of the problem itself explains a way to verify that intermediate results are correct.

> Did you have an estimation of the date?

Yes and that one is easy: I wrote a little piece of code (in Clojure, whatever, it's unrelated to finding the solution) which would check at which intermediate result the computation was and then compute an "ETA".

Fun story: I'd regularly send my brother something like this...

{ :time-left-in-years 1.239341704718417 :eta "1 year 2 month(s) 26 day(s)" :percentage-done 0.6284244211583109 }

(that's an actual estimate I found in my old emails)

: )