Another magnificent puzzle from the Mango team, and a poignant reminder of the sad death of Roddy Forman.
As I'm hopeless when it comes to working in non-decimal bases I feared I would fail at the last hurdle, but I reckon it's possible to arrive at a correct solution without doing any complex maths, provided one has twigged the theme. Of the four likely thematic dates, three can be eliminated immediately because they fail to satisfy some of the conventional numerical provisions in the puzzle, so that gets 8 number values; assigning values to the remainder is straightforward.
I did manage to do the base d addition as a check (and it all tallied), but it wasn't strictly necessary.