Use the same tag twice for 2 different style indexes

[Wolfgang]

Hi!

I would like to use the tag Date twice in the index: One time formated to year, and one time formated as decade.
Reason: I found formatting to decade dismisses the option to filter down by year. So with one Date index given, you either have the choice to see decade chapters _or_ years chapters. If I could bring in the Date tag twice (with different names or something) I could have both options available.

greets,
Wolfgang