diff --git a/__tests__/lib/blog-helper.test.ts b/__tests__/lib/blog-helper.test.ts
index 5b1d6830..87f5ed94 100644
--- a/__tests__/lib/blog-helper.test.ts
+++ b/__tests__/lib/blog-helper.test.ts
@@ -1,10 +1,28 @@
 jest.mock('../../lib/notion/blog-index-cache')
 
 import {
+  getDateStr,
   isYouTubeURL,
   parseYouTubeVideoId,
 } from '../../lib/blog-helpers'
 
+describe('getDateStr', () => {
+  it('returns str when date has time', async () => {
+    const got = getDateStr('2022-12-30T00:09:00.000+09:00')
+    expect(got).toEqual('2022-12-30')
+  })
+
+  it('returns str when date has no time', async () => {
+    const got = getDateStr('2022-12-30')
+    expect(got).toEqual('2022-12-30')
+  })
+
+  it('returns local time str considering timezone', async () => {
+    const got = getDateStr('2022-12-30T00:00:00.000+09:00')
+    expect(got).toEqual('2022-12-30')
+  })
+})
+
 describe('isYouTubeURL', () => {
   it('returns false with not YouTube URL', async () => {
     const got = isYouTubeURL(new URL('https://www.google.com/'))