diff --git a/e2e/vendors/vendor-operations.spec.ts b/e2e/vendors/vendor-operations.spec.ts
index 20799b49..97d23975 100644
--- a/e2e/vendors/vendor-operations.spec.ts
+++ b/e2e/vendors/vendor-operations.spec.ts
@@ -285,6 +285,8 @@ test.describe("Vendor operations", () => {
     await page.getByLabel("Priority").click();
     await page.getByRole("option", { name: "Sev 1" }).click();
     await page.getByLabel("Scheduled").fill("2026-07-25");
+    await page.getByLabel("Appointment start").fill("2026-07-25T10:00");
+    await page.getByLabel("Appointment end").fill("2026-07-25T12:00");
     await page.getByLabel("Company + Technician").click();
     await page.getByRole("option", { name: /Gateway Plumbing · Taylor Tech/ }).click();
     await page.getByLabel("Assignment ETA").fill("2026-07-25T08:30");
@@ -295,8 +297,8 @@ test.describe("Vendor operations", () => {
       .poll(() => assignment)
       .toEqual({
         vendorId: 7,
-        scheduledStart: "2026-07-25T09:00:00.000Z",
-        scheduledEnd: "2026-07-25T11:00:00.000Z",
+        scheduledStart: "2026-07-25T10:00:00.000Z",
+        scheduledEnd: "2026-07-25T12:00:00.000Z",
         estimatedArrivalAt: "2026-07-25T08:30:00.000Z",
         etaManualOverride: true,
       });
@@ -371,6 +373,89 @@ test.describe("Vendor operations", () => {
     await context.close();
   });
 
+  test("converts explicit local schedule bounds to the correct UTC instants across a DST transition", async ({
+    browser,
+  }) => {
+    // America/New_York springs forward on 2026-03-08 at 02:00 EST -> 03:00 EDT.
+    // The window below straddles the transition: 01:00 is EST (UTC-5) while
+    // 04:00 is EDT (UTC-4). Each bound must resolve its own offset, proving no
+    // fixed-window assumption remains.
+    const context = await browser.newContext({ timezoneId: "America/New_York" });
+    const page = await context.newPage();
+    await authenticate(page);
+    let assignment: Record<string, unknown> | undefined;
+
+    await page.route("**/api/Contact/GetLocations", (route) =>
+      fulfillJson(route, { data: [{ Id: 10, Name: "St. Louis HQ" }] }),
+    );
+    await page.route("**/api/Employee/Dropdown", (route) => fulfillJson(route, { data: [] }));
+    await page.route("**/api/DropdownOptions/ByCategory/**", (route) =>
+      fulfillJson(route, { data: [] }),
+    );
+    await page.route("**/api/vendor-operations/availability*", (route) =>
+      fulfillJson(route, {
+        data: [
+          {
+            id: 7,
+            companyName: "Gateway Plumbing",
+            contactName: "Taylor Tech",
+            availabilityStatus: "Available",
+            isPreferred: true,
+            conflicts: [],
+          },
+        ],
+      }),
+    );
+    await page.route("**/api/WorkOrder/AddWorkorder", (route) =>
+      fulfillJson(route, {
+        data: {
+          Id: 902,
+          WorkerOrderTitle: "Boiler repair",
+          LocationId: 10,
+          Priority: "Sev 1",
+          Status: "Open",
+          Comments: [],
+          AuditLog: [],
+          Dispatches: [],
+        },
+      }),
+    );
+    await page.route("**/api/vendor-operations/work-orders/902/assignment", async (route) => {
+      assignment = route.request().postDataJSON() as Record<string, unknown>;
+      await fulfillJson(route, { availabilityStatus: "Available", conflicts: [] });
+    });
+    await page.route("**/api/WorkOrder/GetWorkorderById*", (route) =>
+      fulfillJson(route, {
+        data: {
+          Id: 902,
+          WorkerOrderTitle: "Boiler repair",
+          LocationId: 10,
+          Priority: "Sev 1",
+          Status: "Open",
+          Comments: [],
+          AuditLog: [],
+          Dispatches: [],
+        },
+      }),
+    );
+
+    await page.goto("/workorders/new");
+    await page.getByLabel("Title").fill("Boiler repair");
+    await page.getByLabel("Location").click();
+    await page.getByRole("option", { name: "St. Louis HQ" }).click();
+    await page.getByLabel("Priority").click();
+    await page.getByRole("option", { name: "Sev 1" }).click();
+    await page.getByLabel("Appointment start").fill("2026-03-08T01:00");
+    await page.getByLabel("Appointment end").fill("2026-03-08T04:00");
+    await page.getByLabel("Company + Technician").click();
+    await page.getByRole("option", { name: /Gateway Plumbing · Taylor Tech/ }).click();
+    await page.getByRole("button", { name: "Create" }).click();
+
+    await expect.poll(() => assignment?.scheduledStart).toBe("2026-03-08T06:00:00.000Z");
+    await expect.poll(() => assignment?.scheduledEnd).toBe("2026-03-08T08:00:00.000Z");
+    await context.close();
+  });
+
   test("focuses the Vendor & Time section when arriving from a no-vendor alert", async ({
     page,
   }) => {
@@ -383,7 +468,18 @@ test.describe("Vendor operations", () => {
       fulfillJson(route, { data: [] }),
     );
     await page.route("**/api/vendor-operations/availability*", (route) =>
-      fulfillJson(route, { data: [] }),
+      fulfillJson(route, {
+        data: [
+          {
+            id: 7,
+            companyName: "Gateway Plumbing",
+            contactName: "Taylor Tech",
+            availabilityStatus: "Available",
+            isPreferred: true,
+            conflicts: [],
+          },
+        ],
+      }),
     );
     await page.route("**/api/WorkOrder/GetWorkorderById*", (route) =>
       fulfillJson(route, {
@@ -405,5 +501,8 @@ test.describe("Vendor operations", () => {
     const vendorSection = page.getByRole("region", { name: "Vendor & Time" });
     await expect(vendorSection).toBeVisible();
     await expect(vendorSection).toBeFocused();
+    await page.getByLabel("Company + Technician").click();
+    const option = page.getByRole("option", { name: /Gateway Plumbing · Taylor Tech/ });
+    await expect(option).toContainText("Unknown");
   });
 });
diff --git a/playwright.config.ts b/playwright.config.ts
index 24d54ed6..7360b05c 100644
--- a/playwright.config.ts
+++ b/playwright.config.ts
@@ -10,8 +10,9 @@ export default defineConfig({
   use: {
     // Port 4173 isolates Playwright from the app's normal port 3000 server.
     baseURL: "http://localhost:4173",
-    // Pin the browser timezone to UTC so date-to-ISO assertions (e.g. the fixed
-    // 09:00-11:00 schedule window) are deterministic regardless of host locale.
+    // Pin the browser timezone to UTC so date-to-ISO assertions in the default
+    // create flow are deterministic regardless of host locale. DST-transition
+    // scenarios override this with their own non-UTC context.
     timezoneId: "UTC",
     trace: "on-first-retry",
   },
diff --git a/src/app/(protected)/workorders/_components/vendor-assignment-section.tsx b/src/app/(protected)/workorders/_components/vendor-assignment-section.tsx
index 3df1c9ff..92e55413 100644
--- a/src/app/(protected)/workorders/_components/vendor-assignment-section.tsx
+++ b/src/app/(protected)/workorders/_components/vendor-assignment-section.tsx
@@ -29,6 +29,11 @@ type VendorAndTimeSectionProps = {
   onEtaChange: (value: string) => void;
   etaManualOverride: boolean;
   onOverrideChange: (value: boolean) => void;
+  scheduledStartLocal: string;
+  onScheduledStartChange: (value: string) => void;
+  scheduledEndLocal: string;
+  onScheduledEndChange: (value: string) => void;
+  scheduleBoundsError: string | null;
 };
 
 export function VendorAndTimeSection({
@@ -41,11 +46,40 @@ export function VendorAndTimeSection({
   onEtaChange,
   etaManualOverride,
   onOverrideChange,
+  scheduledStartLocal,
+  onScheduledStartChange,
+  scheduledEndLocal,
+  onScheduledEndChange,
+  scheduleBoundsError,
 }: VendorAndTimeSectionProps) {
+  const hasBoundsError = Boolean(scheduleBoundsError);
   return (
     <Box ref={containerRef} role="region" aria-label="Vendor & Time" tabIndex={-1}>
       <SectionTitle>Vendor & Time</SectionTitle>
       <Stack direction={{ xs: "column", md: "row" }} spacing={2}>
+        <TextField
+          label="Appointment start"
+          type="datetime-local"
+          value={scheduledStartLocal}
+          onChange={(event) => onScheduledStartChange(event.target.value)}
+          error={hasBoundsError}
+          slotProps={{ inputLabel: { shrink: true } }}
+          className="flex-1"
+        />
+        <TextField
+          label="Appointment end"
+          type="datetime-local"
+          value={scheduledEndLocal}
+          onChange={(event) => onScheduledEndChange(event.target.value)}
+          error={hasBoundsError}
+          slotProps={{ inputLabel: { shrink: true } }}
+          className="flex-1"
+        />
+      </Stack>
+      <Text variant="error" when={hasBoundsError} className="mt-1">
+        {scheduleBoundsError}
+      </Text>
+      <Stack direction={{ xs: "column", md: "row" }} spacing={2} className="mt-2">
         <Autocomplete
           options={availableVendors}
           loading={isLoadingVendors}
diff --git a/src/app/(protected)/workorders/_components/work-order-form.tsx b/src/app/(protected)/workorders/_components/work-order-form.tsx
index 524db059..35e3afa7 100644
--- a/src/app/(protected)/workorders/_components/work-order-form.tsx
+++ b/src/app/(protected)/workorders/_components/work-order-form.tsx
@@ -1,4 +1,4 @@
-import { useEffect, useRef, useState } from "react";
+import { useEffect, useMemo, useRef, useState } from "react";
 import { useQuery } from "@tanstack/react-query";
 import { useForm, useWatch } from "react-hook-form";
 import { zodResolver } from "@hookform/resolvers/zod";
@@ -29,6 +29,95 @@ import {
 } from "@/app/(protected)/workorders/_components/work-order-fields";
 import { VendorAndTimeSection } from "@/app/(protected)/workorders/_components/vendor-assignment-section";
 
+type ScheduleBounds = {
+  kind: "valid" | "invalid" | "open";
+  startIso: string | undefined;
+  endIso: string | undefined;
+  error: string | null;
+};
+
+function computeScheduleBounds(startLocal: string, endLocal: string): ScheduleBounds {
+  const hasStart = startLocal.trim() !== "";
+  const hasEnd = endLocal.trim() !== "";
+
+  if (hasStart && hasEnd) {
+    const start = new Date(startLocal);
+    const end = new Date(endLocal);
+    if (
+      !Number.isNaN(start.getTime()) &&
+      !Number.isNaN(end.getTime()) &&
+      end.getTime() > start.getTime()
+    ) {
+      return {
+        kind: "valid",
+        startIso: start.toISOString(),
+        endIso: end.toISOString(),
+        error: null,
+      };
+    }
+    return {
+      kind: "invalid",
+      startIso: undefined,
+      endIso: undefined,
+      error: "Schedule end must be later than schedule start.",
+    };
+  }
+
+  if (hasStart || hasEnd) {
+    return {
+      kind: "invalid",
+      startIso: undefined,
+      endIso: undefined,
+      error: "Provide both a schedule start and end, or leave both blank.",
+    };
+  }
+
+  return { kind: "open", startIso: undefined, endIso: undefined, error: null };
+}
+
+function useVendorSchedule(
+  scheduledStartLocal: string,
+  scheduledEndLocal: string,
+  locationValue: string,
+  tradeValue: string,
+) {
+  const scheduleBounds = useMemo(
+    () => computeScheduleBounds(scheduledStartLocal, scheduledEndLocal),
+    [scheduledStartLocal, scheduledEndLocal],
+  );
+
+  const { data: availableVendors = [], isLoading: isLoadingVendors } = useQuery({
+    queryKey: [
+      "vendor-operations",
+      "availability",
+      scheduleBounds.startIso ?? null,
+      scheduleBounds.endIso ?? null,
+      locationValue,
+      tradeValue,
+    ],
+    queryFn: () =>
+      vendorOperationsApi.availability({
+        start: scheduleBounds.startIso,
+        end: scheduleBounds.endIso,
+        locationId: locationValue || undefined,
+        trade: tradeValue || undefined,
+      }),
+    enabled: scheduleBounds.kind !== "invalid",
+    staleTime: 30_000,
+  });
+
+  const truthfulVendors =
+    scheduleBounds.kind === "valid"
+      ? availableVendors
+      : availableVendors.map((vendor) => ({
+          ...vendor,
+          availabilityStatus: "Unknown" as const,
+          conflicts: [],
+        }));
+
+  return { availableVendors: truthfulVendors, isLoadingVendors, scheduleBounds };
+}
+
 function useWorkOrderFormState() {
   const { id } = useParams();
   const navigate = useNavigate();
@@ -45,6 +134,8 @@ function useWorkOrderFormState() {
   const [selectedVendorId, setSelectedVendorId] = useState<number | null>(null);
   const [estimatedArrivalAt, setEstimatedArrivalAt] = useState("");
   const [etaManualOverride, setEtaManualOverride] = useState(false);
+  const [scheduledStartLocal, setScheduledStartLocal] = useState("");
+  const [scheduledEndLocal, setScheduledEndLocal] = useState("");
 
   const {
     register,
@@ -60,18 +151,13 @@ function useWorkOrderFormState() {
 
   const tradeValue = useWatch({ control, name: "trade" });
   const locationValue = useWatch({ control, name: "locationId" });
-  const scheduledValue = useWatch({ control, name: "scheduledDate" });
-  const { data: availableVendors = [], isLoading: isLoadingVendors } = useQuery({
-    queryKey: ["vendor-operations", "availability", scheduledValue, locationValue, tradeValue],
-    queryFn: () =>
-      vendorOperationsApi.availability({
-        start: scheduledValue ? new Date(`${scheduledValue}T09:00:00`).toISOString() : undefined,
-        end: scheduledValue ? new Date(`${scheduledValue}T11:00:00`).toISOString() : undefined,
-        locationId: locationValue || undefined,
-        trade: tradeValue || undefined,
-      }),
-    staleTime: 30_000,
-  });
+
+  const { availableVendors, isLoadingVendors, scheduleBounds } = useVendorSchedule(
+    scheduledStartLocal,
+    scheduledEndLocal,
+    locationValue,
+    tradeValue,
+  );
 
   const { data: subTrades = [] } = useDropdownOptionsByCategory("SubTrade", {
     parent: tradeValue || undefined,
@@ -108,6 +194,9 @@ function useWorkOrderFormState() {
   }, [assignVendorRequested, isLoading]);
 
   const onSubmit = (values: WorkOrderFormSchemaValues) => {
+    if (scheduleBounds.kind === "invalid") {
+      return;
+    }
     saveMutation.mutate({
       workOrderId: isEdit ? id : undefined,
       values,
@@ -116,12 +205,8 @@ function useWorkOrderFormState() {
           ? undefined
           : {
               vendorId: selectedVendorId,
-              scheduledStart: scheduledValue
-                ? new Date(`${scheduledValue}T09:00:00`).toISOString()
-                : undefined,
-              scheduledEnd: scheduledValue
-                ? new Date(`${scheduledValue}T11:00:00`).toISOString()
-                : undefined,
+              scheduledStart: scheduleBounds.startIso,
+              scheduledEnd: scheduleBounds.endIso,
               estimatedArrivalAt: estimatedArrivalAt
                 ? new Date(estimatedArrivalAt).toISOString()
                 : undefined,
@@ -161,6 +246,12 @@ function useWorkOrderFormState() {
     setEstimatedArrivalAt,
     etaManualOverride,
     setEtaManualOverride,
+    scheduledStartLocal,
+    setScheduledStartLocal,
+    scheduledEndLocal,
+    setScheduledEndLocal,
+    scheduleBoundsError: scheduleBounds.error,
+    isScheduleBoundsInvalid: scheduleBounds.kind === "invalid",
     onSubmit,
     handleCancel,
   };
@@ -194,6 +285,12 @@ export default function WorkOrderFormPage() {
     setEstimatedArrivalAt,
     etaManualOverride,
     setEtaManualOverride,
+    scheduledStartLocal,
+    setScheduledStartLocal,
+    scheduledEndLocal,
+    setScheduledEndLocal,
+    scheduleBoundsError,
+    isScheduleBoundsInvalid,
     onSubmit,
     handleCancel,
   } = useWorkOrderFormState();
@@ -245,6 +342,11 @@ export default function WorkOrderFormPage() {
             onEtaChange={setEstimatedArrivalAt}
             etaManualOverride={etaManualOverride}
             onOverrideChange={setEtaManualOverride}
+            scheduledStartLocal={scheduledStartLocal}
+            onScheduledStartChange={setScheduledStartLocal}
+            scheduledEndLocal={scheduledEndLocal}
+            onScheduledEndChange={setScheduledEndLocal}
+            scheduleBoundsError={scheduleBoundsError}
           />
 
           <ClassificationFields
@@ -270,7 +372,11 @@ export default function WorkOrderFormPage() {
             <Button variant="outlined" onClick={handleCancel} disabled={isSaving}>
               Cancel
             </Button>
-            <Button type="submit" variant="contained" disabled={isSaving}>
+            <Button
+              type="submit"
+              variant="contained"
+              disabled={isSaving || isScheduleBoundsInvalid}
+            >
               {isSaving ? "Saving..." : isEdit ? "Update" : "Create"}
             </Button>
           </Stack>
diff --git a/src/domain/work-orders/use-cases/use-save-work-order.ts b/src/domain/work-orders/use-cases/use-save-work-order.ts
index 98019b72..591031b5 100644
--- a/src/domain/work-orders/use-cases/use-save-work-order.ts
+++ b/src/domain/work-orders/use-cases/use-save-work-order.ts
@@ -23,12 +23,9 @@ type SaveWorkOrderResult = {
 };
 
 class PartialWorkOrderSaveError extends Error {
-  constructor(
-    public readonly workOrderId: string | number,
-    cause: unknown,
-  ) {
-    const detail = cause instanceof Error ? cause.message : "Unknown assignment error";
-    super(`Work order saved, but vendor assignment failed: ${detail}`);
+  constructor(public readonly workOrderId: string | number) {
+    super("Work order saved, but the vendor assignment could not be completed.");
+    this.name = "PartialWorkOrderSaveError";
   }
 }
 
@@ -50,8 +47,8 @@ export function useSaveWorkOrder() {
       if (vendorAssignment) {
         try {
           assignment = await vendorOperationsApi.assignVendor(savedWorkOrder.id, vendorAssignment);
-        } catch (error) {
-          throw new PartialWorkOrderSaveError(savedWorkOrder.id, error);
+        } catch {
+          throw new PartialWorkOrderSaveError(savedWorkOrder.id);
         }
       }
 
@@ -80,7 +77,7 @@ export function useSaveWorkOrder() {
         return;
       }
 
-      toast.error(error.message || "Failed to save work order");
+      toast.error("Unable to save the work order. Please try again.");
     },
   });
 }