diff --git a/packages/api/src/__tests__/timeline-shift-mutation-support.test.ts b/packages/api/src/__tests__/timeline-shift-mutation-support.test.ts
new file mode 100644
index 0000000..c0f21a3
--- /dev/null
+++ b/packages/api/src/__tests__/timeline-shift-mutation-support.test.ts
@@ -0,0 +1,150 @@
+import { describe, expect, it, vi } from "vitest";
+
+const { calculateAllocationMock } = vi.hoisted(() => ({
+  calculateAllocationMock: vi.fn(),
+}));
+
+vi.mock("@capakraken/engine", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("@capakraken/engine")>();
+  return {
+    ...actual,
+    calculateAllocation: calculateAllocationMock,
+  };
+});
+
+import {
+  buildTimelineProjectDateRangeUpdate,
+  buildTimelineProjectShiftAuditChanges,
+  buildTimelineShiftedAssignmentUpdate,
+  recalculateShiftedAssignmentDailyCost,
+} from "../router/timeline-shift-mutation-support.js";
+
+describe("timeline shift mutation support", () => {
+  it("builds shared date range updates for project and demand records", () => {
+    expect(
+      buildTimelineProjectDateRangeUpdate({
+        newStartDate: new Date("2026-04-03T00:00:00.000Z"),
+        newEndDate: new Date("2026-04-12T00:00:00.000Z"),
+      }),
+    ).toEqual({
+      startDate: new Date("2026-04-03T00:00:00.000Z"),
+      endDate: new Date("2026-04-12T00:00:00.000Z"),
+    });
+  });
+
+  it("builds assignment updates with optional recalculated daily cost", () => {
+    expect(
+      buildTimelineShiftedAssignmentUpdate({
+        newStartDate: new Date("2026-04-03T00:00:00.000Z"),
+        newEndDate: new Date("2026-04-12T00:00:00.000Z"),
+        dailyCostCents: 54321,
+      }),
+    ).toEqual({
+      startDate: new Date("2026-04-03T00:00:00.000Z"),
+      endDate: new Date("2026-04-12T00:00:00.000Z"),
+      dailyCostCents: 54321,
+    });
+
+    expect(
+      buildTimelineShiftedAssignmentUpdate({
+        newStartDate: new Date("2026-04-03T00:00:00.000Z"),
+        newEndDate: new Date("2026-04-12T00:00:00.000Z"),
+        dailyCostCents: undefined,
+      }),
+    ).toEqual({
+      startDate: new Date("2026-04-03T00:00:00.000Z"),
+      endDate: new Date("2026-04-12T00:00:00.000Z"),
+    });
+  });
+
+  it("builds project shift audit payloads", () => {
+    expect(
+      buildTimelineProjectShiftAuditChanges({
+        project: {
+          id: "project_1",
+          budgetCents: 200_000,
+          winProbability: 80,
+          startDate: new Date("2026-04-01T00:00:00.000Z"),
+          endDate: new Date("2026-04-10T00:00:00.000Z"),
+        },
+        newStartDate: new Date("2026-04-03T00:00:00.000Z"),
+        newEndDate: new Date("2026-04-12T00:00:00.000Z"),
+        validation: {
+          valid: true,
+          errors: [],
+          warnings: [],
+          conflictDetails: [],
+          costImpact: {
+            currentTotalCents: 100_000,
+            newTotalCents: 112_000,
+            deltaCents: 12_000,
+            budgetCents: 200_000,
+            budgetUtilizationBefore: 50,
+            budgetUtilizationAfter: 56,
+            wouldExceedBudget: false,
+          },
+        },
+      }),
+    ).toEqual({
+      before: {
+        startDate: new Date("2026-04-01T00:00:00.000Z"),
+        endDate: new Date("2026-04-10T00:00:00.000Z"),
+      },
+      after: {
+        startDate: new Date("2026-04-03T00:00:00.000Z"),
+        endDate: new Date("2026-04-12T00:00:00.000Z"),
+      },
+      costImpact: {
+        currentTotalCents: 100_000,
+        newTotalCents: 112_000,
+        deltaCents: 12_000,
+        budgetCents: 200_000,
+        budgetUtilizationBefore: 50,
+        budgetUtilizationAfter: 56,
+        wouldExceedBudget: false,
+      },
+    });
+  });
+
+  it("recalculates daily cost only for staffed assignments", async () => {
+    calculateAllocationMock.mockReturnValueOnce({ dailyCostCents: 54321 });
+
+    await expect(
+      recalculateShiftedAssignmentDailyCost({
+        db: {
+          vacation: { findMany: vi.fn().mockResolvedValue([]) },
+          calculationRule: { findMany: vi.fn().mockResolvedValue([{ id: "rule_1" }]) },
+        } as never,
+        assignment: {
+          resourceId: "resource_1",
+          hoursPerDay: 8,
+          metadata: { includeSaturday: true },
+          resource: {
+            lcrCents: 5_000,
+            availability: {
+              monday: 8,
+              tuesday: 8,
+              wednesday: 8,
+              thursday: 8,
+              friday: 8,
+            },
+          },
+        } as never,
+        newStartDate: new Date("2026-04-03T00:00:00.000Z"),
+        newEndDate: new Date("2026-04-12T00:00:00.000Z"),
+      }),
+    ).resolves.toBe(54321);
+
+    await expect(
+      recalculateShiftedAssignmentDailyCost({
+        db: {} as never,
+        assignment: {
+          resourceId: null,
+          resource: null,
+        } as never,
+        newStartDate: new Date("2026-04-03T00:00:00.000Z"),
+        newEndDate: new Date("2026-04-12T00:00:00.000Z"),
+      }),
+    ).resolves.toBeUndefined();
+  });
+});
diff --git a/packages/api/src/router/timeline-shift-mutation-support.ts b/packages/api/src/router/timeline-shift-mutation-support.ts
new file mode 100644
index 0000000..ae62436
--- /dev/null
+++ b/packages/api/src/router/timeline-shift-mutation-support.ts
@@ -0,0 +1,70 @@
+import type { SplitAssignmentRecord } from "@capakraken/application";
+import { Prisma, type PrismaClient } from "@capakraken/db";
+import type { ShiftValidationResult, WeekdayAvailability } from "@capakraken/shared";
+import { calculateTimelineAllocationDailyCost } from "./timeline-cost-support.js";
+import type { TimelineShiftProjectRecord } from "./timeline-shift-support.js";
+
+export function buildTimelineProjectDateRangeUpdate(input: {
+  newStartDate: Date;
+  newEndDate: Date;
+}) {
+  return {
+    startDate: input.newStartDate,
+    endDate: input.newEndDate,
+  };
+}
+
+export function buildTimelineShiftedAssignmentUpdate(input: {
+  newStartDate: Date;
+  newEndDate: Date;
+  dailyCostCents: number | undefined;
+}) {
+  return {
+    ...buildTimelineProjectDateRangeUpdate(input),
+    ...(input.dailyCostCents !== undefined ? { dailyCostCents: input.dailyCostCents } : {}),
+  };
+}
+
+export function buildTimelineProjectShiftAuditChanges(input: {
+  project: TimelineShiftProjectRecord;
+  newStartDate: Date;
+  newEndDate: Date;
+  validation: ShiftValidationResult;
+}): Prisma.InputJsonValue {
+  return {
+    before: {
+      startDate: input.project.startDate,
+      endDate: input.project.endDate,
+    },
+    after: {
+      startDate: input.newStartDate,
+      endDate: input.newEndDate,
+    },
+    costImpact: input.validation.costImpact,
+  } as unknown as Prisma.InputJsonValue;
+}
+
+export async function recalculateShiftedAssignmentDailyCost(input: {
+  db: PrismaClient;
+  assignment: SplitAssignmentRecord;
+  newStartDate: Date;
+  newEndDate: Date;
+}): Promise<number | undefined> {
+  if (!input.assignment.resourceId || !input.assignment.resource) {
+    return undefined;
+  }
+
+  const metadata = (input.assignment.metadata as Record<string, unknown> | null | undefined) ?? {};
+  const includeSaturday = (metadata.includeSaturday as boolean | undefined) ?? false;
+
+  return calculateTimelineAllocationDailyCost({
+    db: input.db,
+    resourceId: input.assignment.resourceId,
+    lcrCents: input.assignment.resource.lcrCents,
+    hoursPerDay: input.assignment.hoursPerDay,
+    startDate: input.newStartDate,
+    endDate: input.newEndDate,
+    availability: input.assignment.resource.availability as WeekdayAvailability,
+    includeSaturday,
+  });
+}
diff --git a/packages/api/src/router/timeline-shift-support.ts b/packages/api/src/router/timeline-shift-support.ts
index 0c5d28b..2e98542 100644
--- a/packages/api/src/router/timeline-shift-support.ts
+++ b/packages/api/src/router/timeline-shift-support.ts
@@ -4,12 +4,17 @@ import {
   type SplitAssignmentRecord,
   type SplitDemandRequirementRecord,
 } from "@capakraken/application";
-import { Prisma, type PrismaClient } from "@capakraken/db";
+import type { PrismaClient } from "@capakraken/db";
 import { validateShift } from "@capakraken/engine";
 import type { ShiftValidationResult } from "@capakraken/shared";
 import { TRPCError } from "@trpc/server";
-import { calculateTimelineAllocationDailyCost } from "./timeline-cost-support.js";
 import type { TimelineShiftPlan } from "./timeline-shift-planning.js";
+import {
+  buildTimelineProjectDateRangeUpdate,
+  buildTimelineProjectShiftAuditChanges,
+  buildTimelineShiftedAssignmentUpdate,
+  recalculateShiftedAssignmentDailyCost,
+} from "./timeline-shift-mutation-support.js";
 
 export interface TimelineShiftProjectRecord {
   id: string;
@@ -92,49 +97,6 @@ export function buildTimelineProjectShiftEventPayload(input: {
   };
 }
 
-function buildTimelineProjectShiftAuditChanges(input: {
-  project: TimelineShiftProjectRecord;
-  newStartDate: Date;
-  newEndDate: Date;
-  validation: ShiftValidationResult;
-}): Prisma.InputJsonValue {
-  return {
-    before: {
-      startDate: input.project.startDate,
-      endDate: input.project.endDate,
-    },
-    after: {
-      startDate: input.newStartDate,
-      endDate: input.newEndDate,
-    },
-    costImpact: input.validation.costImpact,
-  } as unknown as Prisma.InputJsonValue;
-}
-
-async function recalculateShiftedAssignmentDailyCost(input: {
-  db: PrismaClient;
-  assignment: SplitAssignmentRecord;
-  newStartDate: Date;
-  newEndDate: Date;
-}): Promise<number | undefined> {
-  if (!input.assignment.resourceId || !input.assignment.resource) {
-    return undefined;
-  }
-
-  const metadata = (input.assignment.metadata as Record<string, unknown> | null | undefined) ?? {};
-  const includeSaturday = (metadata.includeSaturday as boolean | undefined) ?? false;
-  return calculateTimelineAllocationDailyCost({
-    db: input.db,
-    resourceId: input.assignment.resourceId,
-    lcrCents: input.assignment.resource.lcrCents,
-    hoursPerDay: input.assignment.hoursPerDay,
-    startDate: input.newStartDate,
-    endDate: input.newEndDate,
-    availability: input.assignment.resource.availability as import("@capakraken/shared").WeekdayAvailability,
-    includeSaturday,
-  });
-}
-
 export async function applyTimelineProjectShift(input: ApplyTimelineProjectShiftInput) {
   const validation = buildTimelineProjectShiftValidation({
     context: input.context,
@@ -146,20 +108,14 @@ export async function applyTimelineProjectShift(input: ApplyTimelineProjectShift
   const updatedProject = await input.db.$transaction(async (tx) => {
     const projectRecord = await tx.project.update({
       where: { id: input.projectId },
-      data: {
-        startDate: input.newStartDate,
-        endDate: input.newEndDate,
-      },
+      data: buildTimelineProjectDateRangeUpdate(input),
     });
 
     for (const demandRequirement of input.context.demandRequirements) {
       await updateDemandRequirement(
         tx as unknown as Parameters<typeof updateDemandRequirement>[0],
         demandRequirement.id,
-        {
-          startDate: input.newStartDate,
-          endDate: input.newEndDate,
-        },
+        buildTimelineProjectDateRangeUpdate(input),
       );
     }
 
@@ -174,11 +130,11 @@ export async function applyTimelineProjectShift(input: ApplyTimelineProjectShift
       await updateAssignment(
         tx as unknown as Parameters<typeof updateAssignment>[0],
         assignment.id,
-        {
-          startDate: input.newStartDate,
-          endDate: input.newEndDate,
-          ...(dailyCostCents !== undefined ? { dailyCostCents } : {}),
-        },
+        buildTimelineShiftedAssignmentUpdate({
+          newStartDate: input.newStartDate,
+          newEndDate: input.newEndDate,
+          dailyCostCents,
+        }),
       );
     }