diff --git a/docs/architecture.md b/docs/architecture.md
index 7676481..53c17c3 100644
--- a/docs/architecture.md
+++ b/docs/architecture.md
@@ -157,6 +157,16 @@ No browser restart recovery of these uncommitted forms is promised. JavaScript
 strings cannot be guaranteed to have been physically erased from process memory.
 Exports are deliberate readable files with manual-recovery limits.
 
+The preparatory Journal habit-detail selector derives streak scalars from validated
+civil completion dates without persisting a streak or retaining source objects.
+A checked selected day ends its consecutive run there; an unchecked day may retain
+the run ending exactly one day earlier. Creation/archive bounds are inclusive,
+and missing or undone dates stop the run. UTC calendar ordinals avoid daylight-
+saving arithmetic across supported years 1000..9999. It indexes completion history
+once and returns fresh scalar projections. The dependent Journal control slice
+owns its unlocked-view lifecycle and visible feedback; this helper alone changes
+no application screen, key, envelope or storage format.
+
 Clips stores validated HTTP(S) links and optional actual pasted text. It does not
 fetch a page as part of local Save link. Duplicate comparison uses a separate
 canonical URL; the validated original URL is retained. Read/read-later/archive
diff --git a/web/src/ui/journalHabitDetails.ts b/web/src/ui/journalHabitDetails.ts
new file mode 100644
index 0000000..3cf7abe
--- /dev/null
+++ b/web/src/ui/journalHabitDetails.ts
@@ -0,0 +1,82 @@
+import type { JournalData } from "../collections/journal";
+
+export type JournalHabitDetail = Readonly<{
+  days: number;
+  through: string | null;
+  completed: boolean;
+}>;
+const DAY_MS = 86400000;
+
+function civilDay(value: string): number {
+  if (!/^[1-9]\d{3}-\d{2}-\d{2}$/.test(value))
+    throw new RangeError("Choose a valid Journal calendar date.");
+  const time = Date.UTC(
+    Number(value.slice(0, 4)),
+    Number(value.slice(5, 7)) - 1,
+    Number(value.slice(8, 10)),
+  );
+  if (
+    !Number.isFinite(time) ||
+    new Date(time).toISOString().slice(0, 10) !== value
+  )
+    throw new RangeError("Choose a valid Journal calendar date.");
+  return time / DAY_MS;
+}
+
+/** Derive only view scalars from an already validated, unlocked Journal image.
+ * No clock, persisted streak, retained source reference or cross-session cache. */
+export function journalHabitDetails(
+  data: Pick<JournalData, "habits" | "completions">,
+  selectedDay: string,
+): ReadonlyMap<string, JournalHabitDetail> {
+  const selected = civilDay(selectedDay);
+  const active = new Map<
+    string,
+    { created: number; completedDays: Set<number> }
+  >();
+  for (const habit of data.habits) {
+    if (
+      habit.createdOn > selectedDay ||
+      (habit.archivedOn && habit.archivedOn < selectedDay)
+    )
+      continue;
+    active.set(habit.id, {
+      created: civilDay(habit.createdOn),
+      completedDays: new Set(),
+    });
+  }
+  // One pass over the protected history, rather than one whole-history filter
+  // per habit. Future and out-of-interval completions cannot extend this run.
+  for (const completion of data.completions) {
+    if (!completion.done || completion.date > selectedDay) continue;
+    const habit = active.get(completion.habitId);
+    if (!habit) continue;
+    const day = civilDay(completion.date);
+    if (day >= habit.created) habit.completedDays.add(day);
+  }
+  const details = new Map<string, JournalHabitDetail>();
+  for (const [id, habit] of active) {
+    const completed = habit.completedDays.has(selected);
+    const end = completed ? selected : selected - 1;
+    let days = 0;
+    // A missing or explicitly undone day ends the streak immediately. The
+    // total walk is bounded by the indexed true completion records plus H.
+    for (
+      let cursor = end;
+      cursor >= habit.created && habit.completedDays.has(cursor);
+      cursor--
+    )
+      days++;
+    details.set(
+      id,
+      Object.freeze({
+        days,
+        through: days
+          ? new Date(end * DAY_MS).toISOString().slice(0, 10)
+          : null,
+        completed,
+      }),
+    );
+  }
+  return details;
+}
diff --git a/web/tests/ui/journal-habit-details.test.ts b/web/tests/ui/journal-habit-details.test.ts
new file mode 100644
index 0000000..f3a358c
--- /dev/null
+++ b/web/tests/ui/journal-habit-details.test.ts
@@ -0,0 +1,196 @@
+// @vitest-environment node
+import { describe, expect, it } from "vitest";
+import type { Habit, JournalData } from "../../src/collections/journal";
+import { journalHabitDetails } from "../../src/ui/journalHabitDetails";
+
+const at = "2026-01-01T00:00:00.000Z";
+function habit(
+  id = "habit",
+  createdOn = "2026-01-01",
+  archivedOn: string | null = null,
+): Habit {
+  return {
+    id,
+    name: "Private habit",
+    order: 0,
+    createdAt: at,
+    createdOn,
+    archivedAt: archivedOn ? at : null,
+    archivedOn,
+    revision: 1,
+  };
+}
+function completion(date: string, done = true, habitId = "habit") {
+  return { date, done, habitId, updatedAt: at };
+}
+type Source = Pick<JournalData, "habits" | "completions">;
+function source(dates: string[], selectedHabit = habit()): Source {
+  return {
+    habits: [selectedHabit],
+    completions: dates.map((day) => completion(day, true, selectedHabit.id)),
+  };
+}
+
+describe("Journal habit details", () => {
+  it("counts a consecutive run ending on the selected completed date", () => {
+    const data = source(["2026-01-01", "2026-01-02", "2026-01-03"]);
+    expect(journalHabitDetails(data, "2026-01-03").get("habit")).toEqual({
+      days: 3,
+      through: "2026-01-03",
+      completed: true,
+    });
+  });
+  it("reproduces checked then unchecked streak carry without counting an undone day", () => {
+    const data = source([
+      "2026-01-01",
+      "2026-01-02",
+      "2026-01-03",
+      "2026-01-04",
+      "2026-01-05",
+    ]);
+    data.completions.push(completion("2026-01-06", true));
+    expect(journalHabitDetails(data, "2026-01-06").get("habit")?.days).toBe(6);
+    data.completions[5]!.done = false;
+    expect(journalHabitDetails(data, "2026-01-06").get("habit")).toEqual({
+      days: 5,
+      through: "2026-01-05",
+      completed: false,
+    });
+  });
+  it("starts the first eligible completion at one and an empty run at zero", () => {
+    expect(journalHabitDetails(source([]), "2026-01-01").get("habit")).toEqual({
+      days: 0,
+      through: null,
+      completed: false,
+    });
+    expect(
+      journalHabitDetails(source(["2026-01-01"]), "2026-01-01").get("habit"),
+    ).toEqual({ days: 1, through: "2026-01-01", completed: true });
+  });
+  it("does not search past a missing or explicitly undone day for an older run", () => {
+    const data = source(["2026-01-01", "2026-01-02", "2026-01-04"]);
+    expect(journalHabitDetails(data, "2026-01-05").get("habit")?.days).toBe(1);
+    expect(journalHabitDetails(data, "2026-01-06").get("habit")).toEqual({
+      days: 0,
+      through: null,
+      completed: false,
+    });
+    data.completions.push(completion("2026-01-03", false));
+    expect(journalHabitDetails(data, "2026-01-04").get("habit")?.days).toBe(1);
+  });
+  it("uses selected history rather than future completions or the machine date", () => {
+    const data = source([
+      "2026-01-01",
+      "2026-01-02",
+      "2026-01-03",
+      "2026-01-04",
+    ]);
+    expect(journalHabitDetails(data, "2026-01-02").get("habit")).toEqual({
+      days: 2,
+      through: "2026-01-02",
+      completed: true,
+    });
+  });
+  it("respects creation/archive inclusively and does not count out-of-interval records", () => {
+    const data = source(
+      ["2026-01-01", "2026-01-02", "2026-01-03", "2026-01-04"],
+      habit("habit", "2026-01-02", "2026-01-03"),
+    );
+    expect(journalHabitDetails(data, "2026-01-01").size).toBe(0);
+    expect(journalHabitDetails(data, "2026-01-02").get("habit")?.days).toBe(1);
+    expect(journalHabitDetails(data, "2026-01-03").get("habit")?.days).toBe(2);
+    expect(journalHabitDetails(data, "2026-01-04").size).toBe(0);
+  });
+  it("keeps histories by identity across renames, and never crosses habits", () => {
+    const data = source(["2026-01-01", "2026-01-02"]);
+    data.habits.push(habit("other"));
+    data.completions.push(completion("2026-01-03", true, "other"));
+    data.habits[0]!.name = "Renamed private habit";
+    const details = journalHabitDetails(data, "2026-01-03");
+    expect(details.get("habit")).toEqual({
+      days: 2,
+      through: "2026-01-02",
+      completed: false,
+    });
+    expect(details.get("other")).toEqual({
+      days: 1,
+      through: "2026-01-03",
+      completed: true,
+    });
+  });
+  it.each([
+    ["2024-02-28", "2024-02-29", "2024-03-01"],
+    ["2025-12-30", "2025-12-31", "2026-01-01"],
+    ["2026-03-07", "2026-03-08", "2026-03-09"],
+    ["2026-10-03", "2026-10-04", "2026-10-05"],
+  ])(
+    "uses civil UTC components across leap/year/DST boundaries: %s",
+    (a, b, c) => {
+      expect(
+        journalHabitDetails(source([a, b, c], habit("habit", a)), c).get(
+          "habit",
+        )?.days,
+      ).toBe(3);
+    },
+  );
+  it("handles the supported year boundaries without wrapping", () => {
+    expect(
+      journalHabitDetails(
+        source([], habit("habit", "1000-01-01")),
+        "1000-01-01",
+      ).get("habit"),
+    ).toEqual({ days: 0, through: null, completed: false });
+    expect(
+      journalHabitDetails(
+        source(["9999-12-30", "9999-12-31"], habit("habit", "9999-12-30")),
+        "9999-12-31",
+      ).get("habit")?.days,
+    ).toBe(2);
+  });
+  it.each([
+    "2026-02-29",
+    "2026-04-31",
+    "0999-12-31",
+    "10000-01-01",
+    "2026-1-01",
+    "not-a-date",
+  ])("rejects an invalid selected civil date: %s", (day) => {
+    expect(() => journalHabitDetails(source([]), day)).toThrow(RangeError);
+  });
+  it("returns only fresh scalar projections without mutating the protected input", () => {
+    const data = source(["2026-01-01"]);
+    Object.freeze(data.habits[0]);
+    Object.freeze(data.completions[0]);
+    Object.freeze(data.habits);
+    Object.freeze(data.completions);
+    Object.freeze(data);
+    const first = journalHabitDetails(data, "2026-01-01"),
+      detail = first.get("habit")!;
+    expect(Object.keys(detail).sort()).toEqual([
+      "completed",
+      "days",
+      "through",
+    ]);
+    expect(Object.isFrozen(detail)).toBe(true);
+    expect(journalHabitDetails(data, "2026-01-01")).not.toBe(first);
+  });
+  it("derives all hundred habits from the maximum legal completion count", () => {
+    const start = Date.UTC(2020, 0, 1);
+    const day = (i: number) =>
+      new Date(start + i * 86400000).toISOString().slice(0, 10);
+    const data: Source = {
+      habits: Array.from({ length: 100 }, (_, i) => habit(String(i), day(0))),
+      completions: Array.from({ length: 100000 }, (_, i) =>
+        completion(day(Math.floor(i / 100)), true, String(i % 100)),
+      ),
+    };
+    const details = journalHabitDetails(data, day(999));
+    expect(details.size).toBe(100);
+    for (const detail of details.values())
+      expect(detail).toEqual({
+        days: 1000,
+        through: day(999),
+        completed: true,
+      });
+  });
+});