diff --git a/src/hooks/useBalanceOverview.test.ts b/src/hooks/useBalanceOverview.test.ts
new file mode 100644
index 0000000..0ad85f7
--- /dev/null
+++ b/src/hooks/useBalanceOverview.test.ts
@@ -0,0 +1,39 @@
+import { describe, it, expect } from "vitest";
+import { computeBalanceDateRange } from "./useBalanceOverview";
+
+const FIXED_TODAY = new Date(2026, 3, 25); // local 2026-04-25
+
+describe("computeBalanceDateRange", () => {
+  it("returns an empty range for 'all'", () => {
+    expect(computeBalanceDateRange("all", FIXED_TODAY)).toEqual({});
+  });
+
+  it("subtracts 90 days for 3M and emits a from-only range", () => {
+    const r = computeBalanceDateRange("3M", FIXED_TODAY);
+    expect(r.to).toBeUndefined();
+    expect(r.from).toBe("2026-01-25");
+  });
+
+  it("subtracts 180 days for 6M", () => {
+    const r = computeBalanceDateRange("6M", FIXED_TODAY);
+    expect(r.from).toBe("2025-10-27");
+  });
+
+  it("subtracts 365 days for 1A", () => {
+    const r = computeBalanceDateRange("1A", FIXED_TODAY);
+    expect(r.from).toBe("2025-04-25");
+  });
+
+  it("subtracts 1095 days for 3A", () => {
+    const r = computeBalanceDateRange("3A", FIXED_TODAY);
+    expect(r.from).toBe("2023-04-26");
+  });
+
+  it("emits ISO-8601 zero-padded month/day", () => {
+    // 2026-01-05 → 3M → 2025-10-07; both fields zero-padded.
+    const today = new Date(2026, 0, 5);
+    const r = computeBalanceDateRange("3M", today);
+    expect(r.from).toMatch(/^\d{4}-\d{2}-\d{2}$/);
+    expect(r.from).toBe("2025-10-07");
+  });
+});
diff --git a/src/hooks/useBalanceOverview.ts b/src/hooks/useBalanceOverview.ts
new file mode 100644
index 0000000..faf5d23
--- /dev/null
+++ b/src/hooks/useBalanceOverview.ts
@@ -0,0 +1,168 @@
+// useBalanceOverview — scoped useReducer hook backing BalancePage.
+//
+// Domain coverage (per spec-plan-bilan.md v2 / Issue #141):
+//   - Time-series for the evolution chart (totals + per-category breakdown)
+//   - Per-account latest snapshot value + period-anchor value (for Δ%)
+//   - Period selector (3M / 6M / 1A / 3A / Tout)
+//   - Chart mode toggle (line / stacked-area)
+//
+// Returns are intentionally OUT of scope here — they ship in Issue #142
+// (Modified Dietz). The accounts table reserves columns for the return
+// metrics with TODO comments.
+
+import { useReducer, useEffect, useCallback } from "react";
+import {
+  getSnapshotTotalsByDate,
+  getSnapshotTotalsByCategoryAndDate,
+  getAccountsLatestSnapshot,
+  getAccountsPeriodAnchor,
+  type SnapshotTotalPoint,
+  type SnapshotCategoryBreakdownPoint,
+  type AccountLatestSnapshot,
+  type AccountPeriodAnchor,
+  type SnapshotDateRange,
+} from "../services/balance.service";
+
+export type BalancePeriod = "3M" | "6M" | "1A" | "3A" | "all";
+export type BalanceChartMode = "line" | "stacked";
+
+interface State {
+  period: BalancePeriod;
+  chartMode: BalanceChartMode;
+  evolutionTotals: SnapshotTotalPoint[];
+  evolutionByCategory: SnapshotCategoryBreakdownPoint[];
+  accountsLatest: AccountLatestSnapshot[];
+  accountsPeriodAnchor: AccountPeriodAnchor[];
+  isLoading: boolean;
+  error: string | null;
+}
+
+type Action =
+  | { type: "SET_PERIOD"; payload: BalancePeriod }
+  | { type: "SET_CHART_MODE"; payload: BalanceChartMode }
+  | { type: "LOAD_START" }
+  | {
+      type: "LOAD_SUCCESS";
+      payload: {
+        evolutionTotals: SnapshotTotalPoint[];
+        evolutionByCategory: SnapshotCategoryBreakdownPoint[];
+        accountsLatest: AccountLatestSnapshot[];
+        accountsPeriodAnchor: AccountPeriodAnchor[];
+      };
+    }
+  | { type: "LOAD_ERROR"; payload: string };
+
+function initialState(): State {
+  return {
+    period: "1A",
+    chartMode: "line",
+    evolutionTotals: [],
+    evolutionByCategory: [],
+    accountsLatest: [],
+    accountsPeriodAnchor: [],
+    isLoading: false,
+    error: null,
+  };
+}
+
+function reducer(state: State, action: Action): State {
+  switch (action.type) {
+    case "SET_PERIOD":
+      return { ...state, period: action.payload };
+    case "SET_CHART_MODE":
+      return { ...state, chartMode: action.payload };
+    case "LOAD_START":
+      return { ...state, isLoading: true, error: null };
+    case "LOAD_SUCCESS":
+      return {
+        ...state,
+        ...action.payload,
+        isLoading: false,
+        error: null,
+      };
+    case "LOAD_ERROR":
+      return { ...state, isLoading: false, error: action.payload };
+    default:
+      return state;
+  }
+}
+
+/**
+ * Pure helper: turn a `BalancePeriod` into a `SnapshotDateRange` anchored on
+ * the supplied `today` (defaults to now). Exported so the unit tests can
+ * exercise the date math without mocking time.
+ *
+ * Period anchor decision (decisions-log #141): we anchor on `today`, not on
+ * the latest snapshot. Aggregators read snapshot rows so the answer is
+ * identical either way, but anchoring on today keeps the chart's right edge
+ * stable as the user enters new snapshots — intuitive UX.
+ */
+export function computeBalanceDateRange(
+  period: BalancePeriod,
+  today: Date = new Date()
+): SnapshotDateRange {
+  if (period === "all") return {};
+  const days =
+    period === "3M" ? 90 : period === "6M" ? 180 : period === "1A" ? 365 : 1095;
+  const from = new Date(today);
+  from.setDate(from.getDate() - days);
+  // Local-civil `YYYY-MM-DD` (matches normalizeSnapshotDate's expectations).
+  const yyyy = from.getFullYear();
+  const mm = String(from.getMonth() + 1).padStart(2, "0");
+  const dd = String(from.getDate()).padStart(2, "0");
+  return { from: `${yyyy}-${mm}-${dd}` };
+}
+
+export interface UseBalanceOverviewResult {
+  state: State;
+  setPeriod: (period: BalancePeriod) => void;
+  setChartMode: (mode: BalanceChartMode) => void;
+  reload: () => Promise<void>;
+}
+
+export function useBalanceOverview(): UseBalanceOverviewResult {
+  const [state, dispatch] = useReducer(reducer, undefined, initialState);
+
+  const load = useCallback(async (period: BalancePeriod) => {
+    dispatch({ type: "LOAD_START" });
+    try {
+      const range = computeBalanceDateRange(period);
+      // Parallel fetches — no inter-dependency between the four queries.
+      const [totals, byCategory, latest, anchors] = await Promise.all([
+        getSnapshotTotalsByDate(range),
+        getSnapshotTotalsByCategoryAndDate(range),
+        getAccountsLatestSnapshot(),
+        getAccountsPeriodAnchor(range),
+      ]);
+      dispatch({
+        type: "LOAD_SUCCESS",
+        payload: {
+          evolutionTotals: totals,
+          evolutionByCategory: byCategory,
+          accountsLatest: latest,
+          accountsPeriodAnchor: anchors,
+        },
+      });
+    } catch (err) {
+      const message = err instanceof Error ? err.message : String(err);
+      dispatch({ type: "LOAD_ERROR", payload: message });
+    }
+  }, []);
+
+  // Reload whenever the period changes (and on mount).
+  useEffect(() => {
+    void load(state.period);
+  }, [state.period, load]);
+
+  const setPeriod = useCallback((period: BalancePeriod) => {
+    dispatch({ type: "SET_PERIOD", payload: period });
+  }, []);
+
+  const setChartMode = useCallback((mode: BalanceChartMode) => {
+    dispatch({ type: "SET_CHART_MODE", payload: mode });
+  }, []);
+
+  const reload = useCallback(() => load(state.period), [load, state.period]);
+
+  return { state, setPeriod, setChartMode, reload };
+}