diff --git a/src/main/engine/ApplyValidationDataService.ts b/src/main/engine/ApplyValidationDataService.ts
new file mode 100644
index 0000000..d6026af
--- /dev/null
+++ b/src/main/engine/ApplyValidationDataService.ts
@@ -0,0 +1,121 @@
+import type { PostData } from './PostEngine';
+
+function resolvePostCreatedAt(post: { createdAt: Date | string }): Date {
+  if (post.createdAt instanceof Date) {
+    return post.createdAt;
+  }
+
+  const parsed = new Date(post.createdAt);
+  return Number.isNaN(parsed.getTime()) ? new Date() : parsed;
+}
+
+export function buildApplyValidationArchives(posts: PostData[]): {
+  allCategories: Set<string>;
+  allTags: Set<string>;
+  years: Map<number, Date>;
+  yearMonths: Map<string, Date>;
+  yearMonthDays: Map<string, Date>;
+} {
+  const allCategories = new Set<string>();
+  const allTags = new Set<string>();
+  const years = new Map<number, Date>();
+  const yearMonths = new Map<string, Date>();
+  const yearMonthDays = new Map<string, Date>();
+
+  for (const post of posts) {
+    for (const category of post.categories || []) allCategories.add(category);
+    for (const tag of post.tags || []) allTags.add(tag);
+
+    const createdAt = resolvePostCreatedAt(post);
+    const updatedAt = post.updatedAt;
+    const year = createdAt.getFullYear();
+    const month = String(createdAt.getMonth() + 1).padStart(2, '0');
+    const day = String(createdAt.getDate()).padStart(2, '0');
+    const ymKey = `${year}/${month}`;
+    const ymdKey = `${year}/${month}/${day}`;
+
+    if (!years.has(year) || updatedAt > years.get(year)!) {
+      years.set(year, updatedAt);
+    }
+    if (!yearMonths.has(ymKey) || updatedAt > yearMonths.get(ymKey)!) {
+      yearMonths.set(ymKey, updatedAt);
+    }
+    if (!yearMonthDays.has(ymdKey) || updatedAt > yearMonthDays.get(ymdKey)!) {
+      yearMonthDays.set(ymdKey, updatedAt);
+    }
+  }
+
+  return {
+    allCategories,
+    allTags,
+    years,
+    yearMonths,
+    yearMonthDays,
+  };
+}
+
+export function selectRequestedPosts(params: {
+  publishedPosts: PostData[];
+  requestedPostIds: Set<string>;
+  requestedPageSlugs: Set<string>;
+}): {
+  requestedSinglePosts: PostData[];
+  requestedPagePosts: PostData[];
+} {
+  const requestedSinglePosts = params.publishedPosts.filter((post) => params.requestedPostIds.has(post.id));
+  const requestedPagePosts = params.publishedPosts.filter((post) => {
+    if (!params.requestedPageSlugs.has(post.slug)) {
+      return false;
+    }
+    const categories = Array.isArray(post.categories) ? post.categories : [];
+    return categories.includes('page');
+  });
+
+  return {
+    requestedSinglePosts,
+    requestedPagePosts,
+  };
+}
+
+export function buildRequestedArchiveMaps(params: {
+  requestedYears: Set<number>;
+  requestedYearMonths: Set<string>;
+  requestedYearMonthDays: Set<string>;
+  years: Map<number, Date>;
+  yearMonths: Map<string, Date>;
+  yearMonthDays: Map<string, Date>;
+}): {
+  requestedYearsMap: Map<number, Date>;
+  requestedYearMonthsMap: Map<string, Date>;
+  requestedYearMonthDaysMap: Map<string, Date>;
+} {
+  const requestedYearsMap = new Map<number, Date>();
+  for (const year of params.requestedYears) {
+    const lastmod = params.years.get(year);
+    if (lastmod) {
+      requestedYearsMap.set(year, lastmod);
+    }
+  }
+
+  const requestedYearMonthsMap = new Map<string, Date>();
+  for (const ym of params.requestedYearMonths) {
+    const lastmod = params.yearMonths.get(ym);
+    if (lastmod) {
+      requestedYearMonthsMap.set(ym, lastmod);
+    }
+  }
+
+  const requestedYearMonthDaysMap = new Map<string, Date>();
+  for (const ymd of params.requestedYearMonthDays) {
+    const lastmod = params.yearMonthDays.get(ymd);
+    if (lastmod) {
+      requestedYearMonthDaysMap.set(ymd, lastmod);
+    }
+  }
+
+  return {
+    requestedYearsMap,
+    requestedYearMonthsMap,
+    requestedYearMonthDaysMap,
+  };
+}
diff --git a/src/main/engine/BlogGenerationEngine.ts b/src/main/engine/BlogGenerationEngine.ts
index 0e29c96..e681674 100644
--- a/src/main/engine/BlogGenerationEngine.ts
+++ b/src/main/engine/BlogGenerationEngine.ts
@@ -39,6 +39,11 @@ import {
   generateSinglePostPages,
   generateTagPages,
 } from './RoutePageGenerationService';
+import {
+  buildApplyValidationArchives,
+  buildRequestedArchiveMaps,
+  selectRequestedPosts,
+} from './ApplyValidationDataService';
 
 const DEFAULT_MAX_POSTS_PER_PAGE = 50;
 const MIN_MAX_POSTS_PER_PAGE = 1;
@@ -568,34 +573,7 @@ export class BlogGenerationEngine {
       const { publishedPosts, publishedListPosts } = await loadPublishedGenerationSets(this.postEngine, listExcludedCategories);
       const generationPostIndex = buildGenerationPostIndex(publishedListPosts);
 
-      const allCategories = new Set<string>();
-      const allTags = new Set<string>();
-      const years = new Map<number, Date>();
-      const yearMonths = new Map<string, Date>();
-      const yearMonthDays = new Map<string, Date>();
-
-      for (const post of publishedListPosts) {
-        for (const category of post.categories || []) allCategories.add(category);
-        for (const tag of post.tags || []) allTags.add(tag);
-
-        const createdAt = resolvePostCreatedAt(post);
-        const updatedAt = post.updatedAt;
-        const year = createdAt.getFullYear();
-        const month = String(createdAt.getMonth() + 1).padStart(2, '0');
-        const day = String(createdAt.getDate()).padStart(2, '0');
-        const ymKey = `${year}/${month}`;
-        const ymdKey = `${year}/${month}/${day}`;
-
-        if (!years.has(year) || updatedAt > years.get(year)!) {
-          years.set(year, updatedAt);
-        }
-        if (!yearMonths.has(ymKey) || updatedAt > yearMonths.get(ymKey)!) {
-          yearMonths.set(ymKey, updatedAt);
-        }
-        if (!yearMonthDays.has(ymdKey) || updatedAt > yearMonthDays.get(ymdKey)!) {
-          yearMonthDays.set(ymdKey, updatedAt);
-        }
-      }
+      const { allCategories, allTags, years, yearMonths, yearMonthDays } = buildApplyValidationArchives(publishedListPosts);
 
       const targetedPlan = buildTargetedValidationPlan({
         initialPlan: missingPathPlan,
@@ -629,38 +607,20 @@ export class BlogGenerationEngine {
         // no-op for applyValidation
       };
 
-      const requestedSinglePosts = publishedPosts.filter((post) => targetedPlan.requestedPostIds.has(post.id));
-      const requestedPagePosts = publishedPosts.filter((post) => {
-        if (!targetedPlan.requestedPageSlugs.has(post.slug)) {
-          return false;
-        }
-        const categories = Array.isArray(post.categories) ? post.categories : [];
-        return categories.includes('page');
+      const { requestedSinglePosts, requestedPagePosts } = selectRequestedPosts({
+        publishedPosts,
+        requestedPostIds: targetedPlan.requestedPostIds,
+        requestedPageSlugs: targetedPlan.requestedPageSlugs,
       });
 
-      const requestedYearsMap = new Map<number, Date>();
-      for (const year of targetedPlan.requestedYears) {
-        const lastmod = years.get(year);
-        if (lastmod) {
-          requestedYearsMap.set(year, lastmod);
-        }
-      }
-
-      const requestedYearMonthsMap = new Map<string, Date>();
-      for (const ym of targetedPlan.requestedYearMonths) {
-        const lastmod = yearMonths.get(ym);
-        if (lastmod) {
-          requestedYearMonthsMap.set(ym, lastmod);
-        }
-      }
-
-      const requestedYearMonthDaysMap = new Map<string, Date>();
-      for (const ymd of targetedPlan.requestedYearMonthDays) {
-        const lastmod = yearMonthDays.get(ymd);
-        if (lastmod) {
-          requestedYearMonthDaysMap.set(ymd, lastmod);
-        }
-      }
+      const { requestedYearsMap, requestedYearMonthsMap, requestedYearMonthDaysMap } = buildRequestedArchiveMaps({
+        requestedYears: targetedPlan.requestedYears,
+        requestedYearMonths: targetedPlan.requestedYearMonths,
+        requestedYearMonthDays: targetedPlan.requestedYearMonthDays,
+        years,
+        yearMonths,
+        yearMonthDays,
+      });
 
       onProgress(
         48,
diff --git a/tests/engine/ApplyValidationDataService.test.ts b/tests/engine/ApplyValidationDataService.test.ts
new file mode 100644
index 0000000..2e754f8
--- /dev/null
+++ b/tests/engine/ApplyValidationDataService.test.ts
@@ -0,0 +1,76 @@
+import { describe, expect, it } from 'vitest';
+import type { PostData } from '../../src/main/engine/PostEngine';
+import {
+  buildApplyValidationArchives,
+  buildRequestedArchiveMaps,
+  selectRequestedPosts,
+} from '../../src/main/engine/ApplyValidationDataService';
+
+function makePost(overrides: Partial<PostData> = {}): PostData {
+  const createdAt = overrides.createdAt ?? new Date('2025-01-15T10:00:00.000Z');
+  return {
+    id: overrides.id ?? 'post-1',
+    projectId: overrides.projectId ?? 'project',
+    title: overrides.title ?? 'Title',
+    slug: overrides.slug ?? 'title',
+    excerpt: overrides.excerpt,
+    content: overrides.content ?? 'Body',
+    status: overrides.status ?? 'published',
+    author: overrides.author,
+    createdAt,
+    updatedAt: overrides.updatedAt ?? createdAt,
+    publishedAt: overrides.publishedAt,
+    tags: overrides.tags ?? [],
+    categories: overrides.categories ?? [],
+  };
+}
+
+describe('ApplyValidationDataService', () => {
+  it('builds category tag and date archives from list posts', () => {
+    const posts = [
+      makePost({ id: '1', categories: ['news'], tags: ['t1'], createdAt: new Date('2025-01-15T00:00:00.000Z') }),
+      makePost({ id: '2', categories: ['page'], tags: [], createdAt: new Date('2025-02-20T00:00:00.000Z') }),
+    ];
+
+    const result = buildApplyValidationArchives(posts);
+
+    expect(result.allCategories.has('news')).toBe(true);
+    expect(result.allCategories.has('page')).toBe(true);
+    expect(result.allTags.has('t1')).toBe(true);
+    expect(result.years.has(2025)).toBe(true);
+    expect(result.yearMonths.has('2025/01')).toBe(true);
+    expect(result.yearMonths.has('2025/02')).toBe(true);
+    expect(result.yearMonthDays.has('2025/01/15')).toBe(true);
+    expect(result.yearMonthDays.has('2025/02/20')).toBe(true);
+  });
+
+  it('selects requested single/page posts and resolves requested date maps', () => {
+    const publishedPosts = [
+      makePost({ id: 'a', slug: 'post-a', categories: ['news'], createdAt: new Date('2025-01-15T00:00:00.000Z') }),
+      makePost({ id: 'b', slug: 'about', categories: ['page'], createdAt: new Date('2025-01-10T00:00:00.000Z') }),
+    ];
+
+    const selected = selectRequestedPosts({
+      publishedPosts,
+      requestedPostIds: new Set(['a']),
+      requestedPageSlugs: new Set(['about']),
+    });
+
+    expect(selected.requestedSinglePosts.map((p) => p.id)).toEqual(['a']);
+    expect(selected.requestedPagePosts.map((p) => p.id)).toEqual(['b']);
+
+    const archives = buildApplyValidationArchives(publishedPosts);
+    const requested = buildRequestedArchiveMaps({
+      requestedYears: new Set([2025]),
+      requestedYearMonths: new Set(['2025/01']),
+      requestedYearMonthDays: new Set(['2025/01/15']),
+      years: archives.years,
+      yearMonths: archives.yearMonths,
+      yearMonthDays: archives.yearMonthDays,
+    });
+
+    expect(requested.requestedYearsMap.has(2025)).toBe(true);
+    expect(requested.requestedYearMonthsMap.has('2025/01')).toBe(true);
+    expect(requested.requestedYearMonthDaysMap.has('2025/01/15')).toBe(true);
+  });
+});