Quarter centuries index for classical music?

[Wolfgang]

I've started tagging my classical recordings also with the year of composition. For this I have included an additional decade index on my tree, built from the compositionyear.decade.index.
This works fine but is not practical in the way that it is too fine of a time step for grouping compositions on the timeline. For this purpose something like quarter centuries or - maybe even better - half centuries would be ideal.
Is there any way to arrange for a bigger time step on the index than the decade?

best regards,
Wolfgang