---
name: dir-lastwritetime-is-blind-to-nested-writes
description: "A directory's LastWriteTime only stamps changes to its OWN entry list — a build writing deep inside leaves it untouched, so a hot pool can read as weeks cold; take max(LastWriteTime) over files instead."
metadata: 
  node_type: memory
  type: reference
  originSessionId: 96b2ffa6-824d-4728-90df-e6fc882aa798
  modified: 2026-08-19T13:05:00.557Z
---

Measured 2026-08-19. Sizing build pools under a golden disk-floor crunch, I reported the main `target/` as "FIFTEEN DAYS COLD" off `(Get-Item -Force $r).LastWriteTime` = `2026-08-04 17:27`. doyle refuted it: he had built into that pool **that morning**. My own check confirmed him — newest nested file `target\.rustc_info.json` at `2026-08-19 06:02:25`, a **15-day** gap from the directory's own stamp.

**Mechanism:** a directory's `LastWriteTime` stamps changes to that directory's OWN entry list (a child created/renamed/deleted directly in it). A build that rewrites files deep in `debug/.fingerprint/...` touches none of the top-level entries, so the root stamp never moves. Cargo re-creating the same paths under existing subdirs is exactly this shape. Same trap in `ls -ld` / `stat` on Unix.

**Use instead**, when the question is "has anything written here recently":
```powershell
Get-ChildItem -Force -Recurse -File $dir | Sort-Object LastWriteTime -Descending | Select-Object -First 1 FullName,LastWriteTime
```
Costs the same walk the recursive size sum already does — take both in one pass.

**The judgement half, which is the part worth keeping:** the wrong stamp did not change the right answer. The pool was still reapable, and doyle's re-derivation of it was a PROCESS CENSUS (zero processes running from `target\debug`, fleet runs from `AppData\Local\spt-core\bin`, no lane claim) — evidence about the CURRENT holder, not about the past. **Liveness questions are answered by who holds it now, never by a timestamp**; a stamp is at best circumstantial and at worst, as here, a 15-day lie. When a conclusion rests on a mtime, find the holder-side evidence before stating it. Kin: [[derived-view-vs-on-disk-record]], [[delivery-confirmation-is-a-claim-too]].