Constant COLD_AFTER_NS 

pub const COLD_AFTER_NS: i64 = NANOS_PER_HOUR; // 3_600_000_000_000i64

A block goes cold once it has aged out of the hour it was partitioned into.

Reusing the partition width means the tier boundary is derived rather than configured: the same instant that stops new rows landing next to this block is the one that stops queries with a default window from reaching it.