diff --git a/apps/quartz-app/.env.example b/apps/quartz-app/.env.example
new file mode 100644
index 00000000..649aebf8
--- /dev/null
+++ b/apps/quartz-app/.env.example
@@ -0,0 +1,3 @@
+# history window config for issue #655
+NEXT_PUBLIC_HISTORY_START_TYPE=rolling
+NEXT_PUBLIC_HISTORY_START_OFFSET_HOURS=48
\ No newline at end of file
diff --git a/apps/quartz-app/src/data/queries.ts b/apps/quartz-app/src/data/queries.ts
index 87d80499..7f6c5f73 100644
--- a/apps/quartz-app/src/data/queries.ts
+++ b/apps/quartz-app/src/data/queries.ts
@@ -2,6 +2,7 @@
 import { QueryFunction, useQuery } from "@tanstack/react-query";
 import { components, operations, paths } from "../types/schema";
 import client from "./apiClient";
+import { getHistoryStartISO } from "../helpers/historyWindow";
 
 // paths
 export const GET_REGIONS = "/{source}/regions";
@@ -46,6 +47,7 @@ export const getGenerationQuery = (
   region: operations["get_historic_timeseries_route__source___region__generation_get"]["parameters"]["path"]["region"]
 ): QueryFunction<components["schemas"]["GetHistoricGenerationResponse"]> => {
   return async ({ meta, signal }) => {
+    const startISO = getHistoryStartISO();
     const { accessToken } = await fetch("/api/token").then((res) => res.json());
     const { data, error } = await client.GET(GET_GENERATION, {
       params: {
@@ -56,6 +58,9 @@ export const getGenerationQuery = (
         query: {
           ...sharedQueryParams,
           resample_minutes: 15,
+          // start of history window for UI
+          // @ts-ignore – schema typing may not include this yet
+          start_datetime_utc: startISO,
         },
       },
       // Add bearer token to headers
@@ -78,6 +83,7 @@ export const getForecastQuery = (
   forecast_horizon_minutes?: number
 ): QueryFunction<components["schemas"]["GetForecastGenerationResponse"]> => {
   return async ({ meta, signal }) => {
+    const startISO = getHistoryStartISO();
     const { accessToken } = await fetch("/api/token").then((res) => res.json());
     const { data, error } = await client.GET(GET_FORECAST, {
       params: {
@@ -90,6 +96,9 @@ export const getForecastQuery = (
           forecast_horizon,
           forecast_horizon_minutes:
             forecast_horizon === "horizon" ? forecast_horizon_minutes : null,
+          // start of history window for UI
+          // @ts-ignore – schema typing may not include this yet
+          start_datetime_utc: startISO,
         },
       },
       // Add bearer token to headers
diff --git a/apps/quartz-app/src/helpers/historyWindow.ts b/apps/quartz-app/src/helpers/historyWindow.ts
new file mode 100644
index 00000000..24021e6f
--- /dev/null
+++ b/apps/quartz-app/src/helpers/historyWindow.ts
@@ -0,0 +1,41 @@
+export type HistoryStartType = "fixed" | "rolling";
+
+/**
+ * Returns the start Date for the UI history window.
+ *  - rolling: now - OFFSET hours
+ *  - fixed:   midnight (UTC) N days ago, where N = round(OFFSET/24), min 1
+ */
+export function getHistoryStart(now = new Date()): Date {
+  const rawType = (
+    process.env.NEXT_PUBLIC_HISTORY_START_TYPE ?? "rolling"
+  ).toLowerCase();
+  const type: HistoryStartType = rawType === "fixed" ? "fixed" : "rolling";
+
+  const offsetStr = process.env.NEXT_PUBLIC_HISTORY_START_OFFSET_HOURS ?? "48";
+  const offsetHours = Number(offsetStr);
+  const safeOffset =
+    Number.isFinite(offsetHours) && offsetHours > 0 ? offsetHours : 48;
+
+  if (type === "fixed") {
+    const days = Math.max(1, Math.round(safeOffset / 24));
+    const midnightTodayUTC = new Date(
+      Date.UTC(
+        now.getUTCFullYear(),
+        now.getUTCMonth(),
+        now.getUTCDate(),
+        0,
+        0,
+        0,
+        0
+      )
+    );
+    midnightTodayUTC.setUTCDate(midnightTodayUTC.getUTCDate() - days);
+    return midnightTodayUTC;
+  }
+
+  // rolling
+  return new Date(now.getTime() - safeOffset * 3600 * 1000);
+}
+
+export const getHistoryStartISO = (now?: Date) =>
+  getHistoryStart(now).toISOString();