I think this would be a tumbling window rather than a true "rolling" tdigest. I suppose you could decrement the buckets, but it gets a little weird as splits can't really be unsplit. The tumbling window one would probably work, though Tdigest is a little weird on merge etc as it's not completely deterministic with respect to ordering and merging (Uddsketch is) so it's likely you get something that is more than good enough, but wouldn't be the same as if you just calculated it directly so it gets a little confusing and difficult.

(NB: Post author here).