diff --git a/apps/main/package.json b/apps/main/package.json
index e8bebfe..3622986 100644
--- a/apps/main/package.json
+++ b/apps/main/package.json
@@ -28,6 +28,7 @@
         "@typescript-eslint/eslint-plugin": "^5.61.0",
         "@typescript-eslint/parser": "^5.61.0",
         "autoprefixer": "^10.4.14",
+        "dayjs": "^1.11.9",
         "eslint": "^8.44.0",
         "eslint-config-prettier": "^8.8.0",
         "eslint-plugin-import": "^2.27.5",
diff --git a/apps/main/src/utils/getAge.test.ts b/apps/main/src/utils/getAge.test.ts
index 9b98459..26569f1 100644
--- a/apps/main/src/utils/getAge.test.ts
+++ b/apps/main/src/utils/getAge.test.ts
@@ -1,10 +1,12 @@
+import dayjs from "dayjs"
+
 import { testing } from "./getAge"
 
 const { birth, getAge, ageInt, ageDecimal, isOverBirthDay } = testing
 
 describe("getAge tests", () => {
     test("birthday to be 2002-07-30", () => {
-        expect(birth).toEqual(new Date("2002-07-30, 00:00:00.000 +09:00"))
+        expect(birth).toEqual(dayjs("2002-07-30 00:00:00.000+09:00"))
     })
 
     test.each<{
@@ -15,91 +17,87 @@ describe("getAge tests", () => {
         date: number
     }>([
         {
-            timestamp: "2022-12-31, 00:00:00.000 +09:00",
+            timestamp: "2022-12-31 00:00:00.000+09:00",
             overBD: true,
             year: 2022,
             monthIndex: 11,
             date: 31,
         },
         {
-            timestamp: "2023-01-01, 00:00:00.000 +09:00",
+            timestamp: "2023-01-01 00:00:00.000+09:00",
             overBD: false,
             year: 2023,
             monthIndex: 0,
             date: 1,
         },
         {
-            timestamp: "2023-07-29, 00:00:00.000 +09:00",
+            timestamp: "2023-07-29 00:00:00.000+09:00",
             overBD: false,
             year: 2023,
             monthIndex: 6,
             date: 29,
         },
         {
-            timestamp: "2023-07-30, 00:00:00.000 +09:00",
+            timestamp: "2023-07-30 00:00:00.000+09:00",
             overBD: true,
             year: 2023,
             monthIndex: 6,
             date: 30,
         },
         {
-            timestamp: "2023-07-31, 00:00:00.000 +09:00",
+            timestamp: "2023-07-31 00:00:00.000+09:00",
             overBD: true,
             year: 2023,
             monthIndex: 6,
             date: 31,
         },
         {
-            timestamp: "2023-12-31, 00:00:00.000 +09:00",
+            timestamp: "2023-12-31 00:00:00.000+09:00",
             overBD: true,
             year: 2023,
             monthIndex: 11,
             date: 31,
         },
         {
-            timestamp: "2024-01-01, 00:00:00.000 +09:00",
+            timestamp: "2024-01-01 00:00:00.000+09:00",
             overBD: false,
             year: 2024,
             monthIndex: 0,
             date: 1,
         },
     ])("isOverBirthDay to work ($timestamp)", (testData) => {
-        const date = new Date(testData.timestamp)
+        const date = dayjs(testData.timestamp)
 
-        expect(date.getFullYear()).toEqual(testData.year)
-        expect(date.getMonth()).toEqual(testData.monthIndex)
-        expect(date.getDate()).toEqual(testData.date)
+        expect(date.year()).toEqual(testData.year)
+        expect(date.month()).toEqual(testData.monthIndex)
+        expect(date.date()).toEqual(testData.date)
         expect(isOverBirthDay(date)).toEqual(testData.overBD)
     })
 
     test("ageInt to work", () => {
         expect(ageInt(birth)).toEqual(0)
-        expect(ageInt(new Date("2023-07-29"))).toEqual(20)
-        expect(ageInt(new Date("2023-07-30"))).toEqual(21)
-        expect(ageInt(new Date("2023-07-31"))).toEqual(21)
+        expect(ageInt(dayjs("2023-07-29"))).toEqual(20)
+        expect(ageInt(dayjs("2023-07-30"))).toEqual(21)
+        expect(ageInt(dayjs("2023-07-31"))).toEqual(21)
     })
 
     test("ageDecimal to work", () => {
-        expect(ageDecimal(new Date("2023-07-29, 00:00:00.000 +09:00"))).toEqual(
+        expect(ageDecimal(dayjs("2023-07-29 00:00:00.000+09:00"))).toEqual(
             0.9972602739726028
         )
-        expect(ageDecimal(new Date("2023-07-30, 00:00:00.000 +09:00"))).toEqual(
-            0
-        )
-        expect(ageDecimal(new Date("2023-07-31, 00:00:00.000 +09:00"))).toEqual(
+        expect(ageDecimal(dayjs("2023-07-30 00:00:00.000+09:00"))).toEqual(0)
+        expect(ageDecimal(dayjs("2023-07-31 00:00:00.000+09:00"))).toEqual(
             0.00273224043715847
         )
     })
 
     test("getAge to work", () => {
-        expect(getAge(new Date("2002-07-30, 00:00:00.000 +09:00"))).toEqual(0.0)
-        expect(getAge(new Date("2023-07-29, 00:00:00.000 +09:00"))).toEqual(
+        expect(getAge(dayjs("2002-07-30 00:00:00.000+09:00"))).toEqual(0.0)
+        expect(getAge(dayjs("2023-07-29 00:00:00.000+09:00"))).toEqual(
             20.997260273972604
         )
-        expect(getAge(new Date("2023-07-30, 00:00:00.000 +09:00"))).toEqual(
-            21.0
-        )
-        expect(getAge(new Date("2023-07-31, 00:00:00.000 +09:00"))).toEqual(
+        expect(getAge(dayjs("2023-07-30 00:00:00.000+09:00"))).toEqual(21.0)
+        expect(getAge(dayjs("2023-07-31 00:00:00.000+09:00"))).toEqual(
             21.002732240437158
         )
     })
diff --git a/apps/main/src/utils/getAge.ts b/apps/main/src/utils/getAge.ts
index 9ddebc6..b2cdbad 100644
--- a/apps/main/src/utils/getAge.ts
+++ b/apps/main/src/utils/getAge.ts
@@ -1,40 +1,40 @@
+import dayjs, { type Dayjs } from "dayjs"
+
 // my birthday in KST :D
-const birth = new Date("2002-07-30, 00:00:00.000 +09:00")
+const birth = dayjs("2002-07-30 00:00:00.000+09:00")
 
 /**
  * Gets developomp's age with decimal precision
  *
  * @param now - current `Date` in KST
  */
-export default function getAge(now: Date = new Date()): number {
+export default function getAge(now: Dayjs = dayjs()): number {
     return ageInt(now) + ageDecimal(now)
 }
 
 /**
  * Calculates the integer component of the age
  */
-function ageInt(now: Date): number {
-    return (
-        now.getFullYear() - birth.getFullYear() - (isOverBirthDay(now) ? 0 : 1)
-    )
+function ageInt(now: Dayjs): number {
+    return now.year() - birth.year() - (isOverBirthDay(now) ? 0 : 1)
 }
 
 /**
  * Calculates the decimal component of the age
  */
-function ageDecimal(now: Date): number {
+function ageDecimal(now: Dayjs): number {
     // millisecond timestamp of my last birthday
-    const BDPrev = new Date(birth).setFullYear(
-        now.getFullYear() - (isOverBirthDay(now) ? 0 : 1)
-    )
+    const BDPrev = birth
+        .year(now.year() - (isOverBirthDay(now) ? 0 : 1))
+        .valueOf()
 
     // millisecond timestamp of my upcoming birthday
-    const BDNext = new Date(birth).setFullYear(
-        now.getFullYear() + (isOverBirthDay(now) ? 1 : 0)
-    )
+    const BDNext = birth
+        .year(now.year() + (isOverBirthDay(now) ? 1 : 0))
+        .valueOf()
 
     // milliseconds since my last birthday
-    const msSinceLastBD = now.getTime() - BDPrev
+    const msSinceLastBD = now.valueOf() - BDPrev
 
     return msSinceLastBD / (BDNext - BDPrev)
 }
@@ -45,11 +45,10 @@ function ageDecimal(now: Date): number {
  * ...| Dec 31 | Jan 1 | ... | July 29 | Jul 30 | July 31 | ... | Dec 31 | Jan 1 | ...
  * ...|  true  | false | ... |  false  |  true  |  true   | ... |  true  | false | ...
  */
-function isOverBirthDay(now: Date): boolean {
-    if (birth.getMonth() < now.getMonth()) return true
+function isOverBirthDay(now: Dayjs): boolean {
+    if (birth.month() < now.month()) return true
 
-    if (birth.getMonth() === now.getMonth() && birth.getDate() <= now.getDate())
-        return true
+    if (birth.month() === now.month() && birth.date() <= now.date()) return true
 
     return false
 }
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 567e373..8ed1ff8 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -188,6 +188,9 @@ importers:
       autoprefixer:
         specifier: ^10.4.14
         version: 10.4.14(postcss@8.4.25)
+      dayjs:
+        specifier: ^1.11.9
+        version: 1.11.9
       eslint:
         specifier: ^8.44.0
         version: 8.44.0
@@ -3242,6 +3245,10 @@ packages:
       whatwg-url: 12.0.1
     dev: false
 
+  /dayjs@1.11.9:
+    resolution: {integrity: sha512-QvzAURSbQ0pKdIye2txOzNaHmxtUBXerpY0FJsFXUMKbIZeFm5ht1LS/jFsrncjnmtv8HsG0W2g6c0zUjZWmpA==}
+    dev: true
+
   /debug@2.6.9:
     resolution: {integrity: sha512-bC7ElrdJaJnPbAP+1EotYvqZsb3ecl5wi6Bfi6BJTUcNowp6cvspg0jXznRTKDjm/E7AdgFBVeAPVMNcKGsHMA==}
     peerDependencies: