diff --git a/.kiro/specs/build-time-versioning/design.md b/.kiro/specs/build-time-versioning/design.md
index d45682af..2f4194b0 100644
--- a/.kiro/specs/build-time-versioning/design.md
+++ b/.kiro/specs/build-time-versioning/design.md
@@ -60,7 +60,7 @@ Two independent **computation sites** (respecting the existing sandboxed-vs-ambi
 ```mermaid
 graph TB
     DevScript[xce-build.sh git plus date]
-    NixPkg[xce.nix xcecli.nix self.shortRev plus builtins.currentTime]
+    NixPkg[xce.nix xcecli.nix self.shortRev plus preConfigure date]
     MesonOpts[meson commit_id and build_time options]
     CDefine[C VERSION_COMMIT_ID and VERSION_BUILD_TIME defines]
     GoLdflags[go_build ldflags -X injection]
@@ -102,7 +102,7 @@ graph TB
 | Go CLI (xcecli) | Cobra (existing) | `rootCmd.Version` field (built-in `--version`) | Zero new parsing code |
 | Python build | Meson `custom_target` (existing) | New generated build-info module written before `pip install` | `meson-build.sh` |
 | Python CLI (xceadmin) | Click (existing) | `click.version_option()` (built-in `--version`) | Zero new parsing code |
-| Nix packaging | Nix flake (existing) | `self.shortRev`/`self.dirtyShortRev` (existing) + new `builtins.currentTime`-derived value | `xce.nix`, `xcecli.nix` |
+| Nix packaging | Nix flake (existing) | `self.shortRev`/`self.dirtyShortRev` (existing, eval-time) + a `preConfigure` build-phase `date -u` hook for build_time (task 2.2 round 2: `builtins.currentTime` was tried first but breaks pure-eval `nix build`, i.e. requires `--impure`, and yields non-UTC epoch seconds — rejected in review, corrected to a build-phase computation) | `xce.nix`, `xcecli.nix` |
 
 ## File Structure Plan
 
@@ -113,7 +113,7 @@ graph TB
 - `src/controller/main.c`, `src/scheduler/src/main.c` — update `argp_program_version` construction to include build time; add the startup INFO log line (each already has one startup INFO call site to extend/follow).
 - `src/worker/src/main.c` — add a `--version` long option + case (prints the same shape, exits 0); add the startup INFO log line (follows its own existing startup INFO call site).
 - `src/tools/xce-build.sh` — compute `commit_id` (git, `-dirty` suffixed) and `build_time` (UTC `date`) and pass both via new `-Dcommit_id=`/`-Dbuild_time=` entries in `MESON_OPTS`, falling back to `"unknown"` if `git` fails (e.g. not a git checkout).
-- `src/flake.nix`, `src/nixSupport/xce.nix`, `src/nixSupport/xcecli.nix` (exact filenames per existing commit_id wiring) — add a `builtins.currentTime`-derived build-time value passed the same way `commit_id` already is.
+- `src/nixSupport/xce.nix`, `src/nixSupport/xcecli.nix` — a `preConfigure` build-phase hook runs `date -u +%Y-%m-%dT%H:%M:%SZ` and appends `-Dbuild_time=` to `mesonFlagsArray` (build-time, not eval-time, unlike `commit_id`'s `self.shortRev` wiring in `src/flake.nix`/`src/nixSupport/default.nix`, which is unchanged).
 - `src/tools/go_build` — read `XCE_GIT_COMMIT`/`XCE_BUILD_TIME` env vars (empty-safe) and append `-ldflags "-X main.GitCommit=... -X main.BuildTime=..."` to the final `go build` invocation.
 - `src/frontend/apiserver/meson.build`, `src/frontend/xcecli/meson.build` — pass the two new env vars into their `custom_target()` calls (mirrors existing env var passing).
 - `src/frontend/Makefile` — add the same `-ldflags` construction (via `git`/`date` directly, dev-loop path) to `build-server`/`build-cli` targets.
@@ -140,7 +140,7 @@ graph TB
 | 3.1, 3.2 | Startup log line at INFO | controller/scheduler/worker/apiserver startup sequences | existing first-INFO-line call sites, extended | Startup → log |
 | 4.1, 4.2 | Build tooling confirms embedding | `xce-build.sh`, nix packages | non-empty resolved value check before/after `meson setup`/build | Build → verify |
 | 4.3 | Accurate under every C build profile | `src/meson.build`'s project-wide `add_project_arguments()` | applies uniformly to asan/debug/release | n/a |
-| 5.1, 5.2 | Values reflect actual tree/build event | `xce-build.sh` (git+date at invocation time), nix (`self.shortRev`+`builtins.currentTime` at evaluation time) | no hardcoded/stale values | n/a |
+| 5.1, 5.2 | Values reflect actual tree/build event | `xce-build.sh` (git+date at invocation time), nix (`self.shortRev` at evaluation time + `preConfigure` `date -u` at build time) | no hardcoded/stale values | n/a |
 
 ## Components and Interfaces
 
diff --git a/.kiro/specs/build-time-versioning/research.md b/.kiro/specs/build-time-versioning/research.md
index 497f626f..6f0f880d 100644
--- a/.kiro/specs/build-time-versioning/research.md
+++ b/.kiro/specs/build-time-versioning/research.md
@@ -33,10 +33,11 @@
 - **Alternatives Considered**:
   1. Make `xce-build.sh` also avoid shelling out to git, deriving everything from Nix like the packages do — rejected, `xce-build.sh` doesn't invoke Nix package builds at all, it drives `meson` directly inside `nix develop`.
   2. Add a git-invoking `run_command()` inside `meson.build` itself, used by both paths — rejected, breaks the sandboxed `nix build` path where git is unavailable by design.
-- **Selected Approach**: extend the existing split — `xce-build.sh` computes via `git`+`date` and passes `-Dcommit_id=`/`-Dbuild_time=`; the nix packages (`xce.nix`, `xcecli.nix`) compute via `self.shortRev`/`self.dirtyShortRev` (unchanged) + a new `builtins.currentTime`-derived value, passed the same way.
+- **Selected Approach**: extend the existing split — `xce-build.sh` computes via `git`+`date` and passes `-Dcommit_id=`/`-Dbuild_time=`; the nix packages (`xce.nix`, `xcecli.nix`) compute `commit_id` via `self.shortRev`/`self.dirtyShortRev` (unchanged, eval-time) and `build_time` via a `preConfigure` build-phase `date -u` hook appending to `mesonFlagsArray` (build-time, not eval-time), passed the same `-Dbuild_time=` way.
 - **Rationale**: matches the codebase's own existing precedent exactly; zero new dependencies; keeps the sandboxed build's reproducibility model intact.
 - **Trade-offs**: two computation sites to keep in sync (same risk the existing `commit_id`-only mechanism already carries and has lived with).
 - **Follow-up**: verify during implementation that a plain `meson setup` with neither `xce-build.sh` nor a nix package (e.g. `meson setup build` run by hand) degrades to the "unknown" fallback (Requirement 1 AC4), not a build failure.
+- **Correction (task 2.2, round 1 → round 2)**: the nix-side value was originally attempted via `builtins.currentTime` (eval-time) as noted above — this broke Nix's flake pure-eval mode entirely (`nix build .#xcecore_debug`, the documented no-flag command, failed with `error: attribute 'currentTime' missing`, requiring `--impure`) and produced a raw epoch-seconds string rather than UTC ISO-8601. Caught by task-local review (REJECTED, round 1) before merge; corrected to the `preConfigure`/`date -u` build-phase approach described in "Selected Approach" above, verified to restore pure-eval and produce a correctly-formatted value (round 2, APPROVED). Recorded here so this decision's rationale stays accurate for anyone reading only `research.md`.
 
 ### Decision: Worker gets a hand-rolled `--version` case, not a switch to argp
 - **Context**: worker uses `getopt_long()`, not `argp`; controller/scheduler's free `--version` comes from argp's own auto-generated handling.
diff --git a/.kiro/specs/build-time-versioning/tasks.md b/.kiro/specs/build-time-versioning/tasks.md
index 8a17051f..1384eb19 100644
--- a/.kiro/specs/build-time-versioning/tasks.md
+++ b/.kiro/specs/build-time-versioning/tasks.md
@@ -17,10 +17,10 @@
   - _Requirements: 1.1, 1.2, 1.3, 1.4, 4.1, 4.2, 5.1, 5.2_
   - _Boundary: xce-build.sh_
 
-- [ ] 2.2 (P) Nix packages compute and pass build time
-  - Add a `builtins.currentTime`-derived build-time value in `src/flake.nix`, passed via a new `-Dbuild_time=` entry alongside the existing `-Dcommit_id=` wiring in `xce.nix`/`xcecli.nix`
-  - Since `builtins.currentTime` cannot itself be empty, the confirmation this requirement asks for is structural here: assert (in the derivation or a post-build check) that the meson option actually reached the binary, rather than a runtime empty-value check
-  - Observable: `nix build .#xcecore_debug` produces a binary whose `--version` shows a non-empty build time, without any `git` invocation inside the sandboxed derivation
+- [x] 2.2 (P) Nix packages compute and pass build time
+  - ~~Add a `builtins.currentTime`-derived build-time value in `src/flake.nix`~~ — corrected during review (round 1 REJECTED): `builtins.currentTime` breaks pure-eval `nix build` (requires `--impure`) and yields non-UTC epoch seconds. Implemented instead as a `preConfigure` build-phase `date -u +%Y-%m-%dT%H:%M:%SZ` hook in `xce.nix`/`xcecli.nix` appending to `mesonFlagsArray`, alongside the unchanged eval-time `-Dcommit_id=` wiring
+  - Fail loudly (non-empty guard) if `date -u` ever produces empty output, before the value reaches `mesonFlagsArray`
+  - Observable: `nix build .#xcecore_debug` produces a binary whose `--version` shows a non-empty, ISO-8601 UTC build time, without any `git` invocation inside the sandboxed derivation, and without requiring `--impure`
   - _Requirements: 1.1, 1.2, 1.4, 4.1, 4.2, 5.1, 5.2_
   - _Boundary: Nix packaging_
 
@@ -122,3 +122,6 @@
   - Observable: the installed wheel's `--version` output differs from an editable (`pip install -e .`) install's `unknown`-only output, proving the generated module actually replaces the fallback
   - _Requirements: 1.1, 1.2, 2.1, 4.1, 4.2_
   - _Depends: 5.1, 5.2_
+
+## Implementation Notes
+- **Task 2.2**: `builtins.currentTime` cannot be used for anything feeding a `nix build` derivation without `--impure` — Nix's flake pure-eval mode strips it entirely, breaking the plain, documented `nix build .#xcecore`/`.#xcecore_debug` commands. It also yields a raw epoch-seconds integer, not UTC ISO-8601. Any nix-side build metadata that must vary per-build (timestamps, etc.) belongs in a derivation's `preConfigure` (or similar) build-phase shell hook using ordinary coreutils (`date -u`), appended to `mesonFlagsArray` — never in eval-time Nix expressions. `self.shortRev`/`self.dirtyShortRev` remain the correct, eval-time-safe mechanism specifically for the commit id (Nix computes those specially even under pure-eval), so this distinction is per-value, not a blanket "avoid nix expressions" rule.
diff --git a/src/flake.nix b/src/flake.nix
index 198ed8e7..836d0cb7 100644
--- a/src/flake.nix
+++ b/src/flake.nix
@@ -16,6 +16,19 @@
         if builtins.hasAttr "shortRev" self
         then self.shortRev
         else self.dirtyShortRev;
+      # NOTE (build-time-versioning task 2.2, remediation round): unlike
+      # myCommitId (a genuinely flake-level, eval-time concept sourced from
+      # self.shortRev/self.dirtyShortRev), the build timestamp is NOT
+      # computed here. An earlier attempt used `builtins.toString
+      # builtins.currentTime` at this exact spot -- but builtins.currentTime
+      # is stripped entirely under a flake's default pure evaluation
+      # ("error: attribute 'currentTime' missing"), so `nix build
+      # .#xcecore_debug` (no --impure, the documented canonical command)
+      # failed at evaluation. Fixed by moving the build-time computation
+      # into each derivation's own build phase instead (xce.nix/xcecli.nix
+      # `preConfigure`, via stdenv's bundled `date -u`) -- see those files.
+      # Nothing build-time-related needs to be threaded through the flake
+      # or nixSupport/default.nix any more; only myCommitId still does.
     in
       flake-utils.lib.eachSystem supportedSystems (
         system: let
diff --git a/src/nixSupport/default.nix b/src/nixSupport/default.nix
index 5d3dd4d9..d4a65bdf 100644
--- a/src/nixSupport/default.nix
+++ b/src/nixSupport/default.nix
@@ -4,6 +4,12 @@
   ...
 }:
 let
+  # NOTE (build-time-versioning task 2.2, remediation round): build-time is
+  # no longer threaded through here -- it's computed inside each
+  # derivation's own build phase (xce.nix/xcecli.nix `preConfigure`), since
+  # builtins.currentTime is unavailable under flake pure-eval. Only
+  # myCommitId (a genuine eval-time/flake-level value) still needs to be
+  # passed down.
   xcecore = pkgs.callPackage ./xce.nix {
     inherit pkgs myCommitId;
     withDebug = false;
diff --git a/src/nixSupport/xce.nix b/src/nixSupport/xce.nix
index 4f1b16d5..c2d87749 100644
--- a/src/nixSupport/xce.nix
+++ b/src/nixSupport/xce.nix
@@ -103,6 +103,27 @@ stdenv.mkDerivation {
 
   impureEnvVars = [ "HOME" ];
 
+  # build-time-versioning task 2.2 (remediation round): build_time is
+  # computed HERE, in the build phase, not at flake-evaluation time --
+  # builtins.currentTime is unavailable under flake pure-eval (see
+  # flake.nix's note), so `nix build .#xcecore_debug` with no --impure
+  # would fail at evaluation if it were used. `date` (stdenv's bundled
+  # coreutils) is always available inside the build sandbox regardless of
+  # pure-eval -- no --impure, no git, no network needed. Matches
+  # xce-build.sh's (task 2.1) exact ISO-8601 format. mesonConfigurePhase
+  # (nixpkgs meson setup-hook.sh) runs `runHook preConfigure` before it
+  # reads mesonFlags/mesonFlagsArray to build the `meson setup` command
+  # line, so appending to mesonFlagsArray here is picked up at configure
+  # time.
+  preConfigure = ''
+    buildTime="$(date -u +%Y-%m-%dT%H:%M:%SZ)"
+    if [ -z "$buildTime" ]; then
+      echo "xce.nix: date -u produced an empty build time -- the -Dbuild_time= meson option would not reach the binary" >&2
+      exit 1
+    fi
+    mesonFlagsArray+=("-Dbuild_time=$buildTime")
+  '';
+
   mesonFlags =[
     (lib.optional withDebug "--buildtype=debug")
     (lib.optional withAsan "-Db_sanitize=address")
diff --git a/src/nixSupport/xcecli.nix b/src/nixSupport/xcecli.nix
index 981b3f2f..0e7256b3 100644
--- a/src/nixSupport/xcecli.nix
+++ b/src/nixSupport/xcecli.nix
@@ -25,6 +25,17 @@ stdenv.mkDerivation {
 
   impureEnvVars = [ "HOME" ];
 
+  # See xce.nix for why build_time is computed here (build phase) rather
+  # than at flake-evaluation time.
+  preConfigure = ''
+    buildTime="$(date -u +%Y-%m-%dT%H:%M:%SZ)"
+    if [ -z "$buildTime" ]; then
+      echo "xcecli.nix: date -u produced an empty build time -- the -Dbuild_time= meson option would not reach the binary" >&2
+      exit 1
+    fi
+    mesonFlagsArray+=("-Dbuild_time=$buildTime")
+  '';
+
   mesonFlags = [
     "-Dcomponent=xcecli"
     ("-Dcommit_id=" + myCommitId)