diff --git a/build.gradle b/build.gradle
index 95fe2bd..7060adb 100644
--- a/build.gradle
+++ b/build.gradle
@@ -3,6 +3,9 @@ plugins {
     id 'org.springframework.boot' version '3.5.8'
     id 'io.spring.dependency-management' version '1.1.7'
     id 'com.google.protobuf' version '0.9.4'
+    id 'org.jetbrains.kotlin.jvm' version '2.0.21'
+    id 'org.jetbrains.kotlin.plugin.spring' version '2.0.21'
+    id 'org.jetbrains.kotlin.plugin.jpa' version '2.0.21'
 }
 
 group = 'me'
@@ -15,6 +18,10 @@ java {
     }
 }
 
+kotlin {
+    jvmToolchain(21)
+}
+
 repositories {
     mavenCentral()
 }
@@ -36,6 +43,12 @@ dependencies {
     testImplementation 'org.springframework.amqp:spring-rabbit-test'
     testRuntimeOnly 'org.junit.platform:junit-platform-launcher'
 
+    //kotlin
+    implementation 'org.jetbrains.kotlin:kotlin-reflect'
+    implementation 'org.jetbrains.kotlin:kotlin-stdlib'
+    implementation 'com.fasterxml.jackson.module:jackson-module-kotlin'
+    testImplementation 'org.jetbrains.kotlin:kotlin-test-junit5'
+
     // Actuator
     implementation 'org.springframework.boot:spring-boot-starter-actuator'
 
@@ -79,6 +92,12 @@ tasks.named('test') {
 tasks.named('compileJava') {
     dependsOn(tasks.named('generateProto'))
 }
+tasks.named('compileKotlin') {
+    dependsOn(tasks.named('generateProto'))
+}
 tasks.named('compileTestJava') {
     dependsOn(tasks.named('generateTestProto'))
-}
\ No newline at end of file
+}
+tasks.named('compileTestKotlin') {
+    dependsOn(tasks.named('generateTestProto'))
+}
diff --git a/src/test/java/me/pinitnotification/domain/notification/UpcomingScheduleNotificationTest.kt b/src/test/java/me/pinitnotification/domain/notification/UpcomingScheduleNotificationTest.kt
new file mode 100644
index 0000000..b84fa4a
--- /dev/null
+++ b/src/test/java/me/pinitnotification/domain/notification/UpcomingScheduleNotificationTest.kt
@@ -0,0 +1,49 @@
+package me.pinitnotification.domain.notification
+
+import org.assertj.core.api.Assertions
+import org.junit.jupiter.api.Test
+import java.time.Clock
+import java.time.LocalDateTime
+import java.time.OffsetDateTime
+import java.time.ZoneId
+import java.time.ZoneOffset.UTC
+
+internal class UpcomingScheduleNotificationTest {
+    val clock = Clock.fixed(
+        LocalDateTime.of(
+            2026,
+            1,
+            1,
+            0,
+            0,
+            0
+        ).toInstant(UTC), ZoneId.systemDefault()
+    )
+
+    @Test
+    fun updateScheduleStartTime() {
+        //given
+        val notification = getSampleUpcomingScheduleNotification()
+
+        //when
+        notification.updateScheduleStartTime("2024-06-01T10:00:00Z", "123-idempotency-key")
+
+        //then
+        Assertions.assertThat(notification.scheduleStartTime).isEqualTo("2024-06-01T10:00:00Z")
+        Assertions.assertThat(notification.idempotentKey).isEqualTo("123-idempotency-key")
+    }
+
+    @Test
+    fun isDue() {
+        //given
+        val notification = getSampleUpcomingScheduleNotification(
+            scheduleStartTime = "2024-06-01T10:00:00Z"
+        )
+
+        //when
+        val isDue = notification.isDue(OffsetDateTime.now(clock))
+
+        //then
+        Assertions.assertThat(isDue).isTrue()
+    }
+}
\ No newline at end of file
diff --git a/src/test/java/me/pinitnotification/domain/notification/UpcomingScheduleNotificationUtils.kt b/src/test/java/me/pinitnotification/domain/notification/UpcomingScheduleNotificationUtils.kt
new file mode 100644
index 0000000..f1c2ebb
--- /dev/null
+++ b/src/test/java/me/pinitnotification/domain/notification/UpcomingScheduleNotificationUtils.kt
@@ -0,0 +1,29 @@
+package me.pinitnotification.domain.notification
+
+import org.junit.platform.commons.util.ReflectionUtils
+
+fun getSampleUpcomingScheduleNotification(
+    id: Long = 1L,
+    ownerId: Long = 1L,
+    scheduleId: Long = 1L,
+    scheduleTitle: String = "sample",
+    scheduleStartTime: String = "2024-06-01T10:00:00Z",
+    idempotencyKey: String = "",
+): UpcomingScheduleNotification {
+    val sample = UpcomingScheduleNotification(
+        ownerId,
+        scheduleId,
+        scheduleTitle,
+        scheduleStartTime,
+        idempotencyKey
+    )
+    ReflectionUtils.findFields(
+        UpcomingScheduleNotification::class.java,
+        { field -> field.name == "id" },
+        ReflectionUtils.HierarchyTraversalMode.TOP_DOWN
+    ).forEach { field ->
+        field.isAccessible = true
+        field.set(sample, id)
+    }
+    return sample
+}
\ No newline at end of file
diff --git a/src/test/java/me/pinitnotification/domain/push/PushSubscriptionTest.kt b/src/test/java/me/pinitnotification/domain/push/PushSubscriptionTest.kt
new file mode 100644
index 0000000..c982385
--- /dev/null
+++ b/src/test/java/me/pinitnotification/domain/push/PushSubscriptionTest.kt
@@ -0,0 +1,19 @@
+package me.pinitnotification.domain.push
+
+import org.assertj.core.api.Assertions
+import org.junit.jupiter.api.Test
+
+internal class PushSubscriptionTest {
+
+    @Test
+    fun updateToken() {
+        //given
+        val subscription = getSamplePushSubscription()
+
+        //when
+        subscription.updateToken("new-sample-token")
+
+        //then
+        Assertions.assertThat(subscription.token).isEqualTo("new-sample-token")
+    }
+}
\ No newline at end of file
diff --git a/src/test/java/me/pinitnotification/domain/push/PushSubscriptionUtils.kt b/src/test/java/me/pinitnotification/domain/push/PushSubscriptionUtils.kt
new file mode 100644
index 0000000..905cc8f
--- /dev/null
+++ b/src/test/java/me/pinitnotification/domain/push/PushSubscriptionUtils.kt
@@ -0,0 +1,31 @@
+package me.pinitnotification.domain.push
+
+import org.junit.platform.commons.util.ReflectionUtils
+import java.time.Instant
+
+fun getSamplePushSubscription(
+    id: Long = 1L,
+    memberId: Long = 1L,
+    deviceId: String = "sample-device-id",
+    token: String = "sample-token",
+    modifiedAt: Instant = Instant.EPOCH,
+): PushSubscription {
+    val sample = PushSubscription(memberId, deviceId, token)
+    ReflectionUtils.findFields(
+        PushSubscription::class.java,
+        { field -> field.name == "id" },
+        ReflectionUtils.HierarchyTraversalMode.TOP_DOWN
+    ).forEach { field ->
+        field.isAccessible = true
+        field.set(sample, id)
+    }
+    ReflectionUtils.findFields(
+        PushSubscription::class.java,
+        { field -> field.name == "modifiedAt" },
+        ReflectionUtils.HierarchyTraversalMode.TOP_DOWN
+    ).forEach { field ->
+        field.isAccessible = true
+        field.set(sample, modifiedAt)
+    }
+    return sample
+}
\ No newline at end of file