diff --git a/src/main/java/com/process/clash/adapter/persistence/user/userexphistory/UserExpHistoryJpaRepository.java b/src/main/java/com/process/clash/adapter/persistence/user/userexphistory/UserExpHistoryJpaRepository.java
index dd0634805..428aeda72 100644
--- a/src/main/java/com/process/clash/adapter/persistence/user/userexphistory/UserExpHistoryJpaRepository.java
+++ b/src/main/java/com/process/clash/adapter/persistence/user/userexphistory/UserExpHistoryJpaRepository.java
@@ -174,7 +174,7 @@ SELECT COALESCE(SUM(earn_exp), 0)
         JOIN battles b ON b.id IN (:battleIds)
             AND ueh.fk_user_id = :userId
             AND ueh.date >= (b.started_at AT TIME ZONE 'Asia/Seoul')::date
-            AND ueh.date < (b.end_at AT TIME ZONE 'Asia/Seoul')::date
+            AND ueh.date <= ((b.end_at - interval '1 microsecond') AT TIME ZONE 'Asia/Seoul')::date
         GROUP BY b.id
     """, nativeQuery = true)
     List<Object[]> findAverageExpForBattles(
diff --git a/src/main/java/com/process/clash/application/compete/rival/battle/service/AcceptBattleService.java b/src/main/java/com/process/clash/application/compete/rival/battle/service/AcceptBattleService.java
index 8919e7b5a..b4995226d 100644
--- a/src/main/java/com/process/clash/application/compete/rival/battle/service/AcceptBattleService.java
+++ b/src/main/java/com/process/clash/application/compete/rival/battle/service/AcceptBattleService.java
@@ -16,7 +16,6 @@
 import org.springframework.transaction.annotation.Transactional;
 
 import java.time.Instant;
-import java.time.ZoneId;
 import java.util.List;
 
 @Service
@@ -29,7 +28,6 @@ public class AcceptBattleService implements AcceptBattleUseCase {
     private final RivalRepositoryPort rivalRepositoryPort;
     private final UserNoticeRepositoryPort userNoticeRepositoryPort;
     private final CompeteRefetchNotifier competeRefetchNotifier;
-    private final ZoneId battleZoneId;
 
     @Override
     public void execute(ModifyBattleData.Command command) {
@@ -39,8 +37,7 @@ public void execute(ModifyBattleData.Command command) {
         Battle battle = battleRepositoryPort.findById(command.id())
                 .orElseThrow(BattleNotFoundException::new);
 
-        // 승인 시각을 배틀 시작 시각으로 사용, 종료 시각은 수락일(KST) + duration일의 자정 KST
-        Battle updatedBattle = battle.accept(Instant.now(), battleZoneId);
+        Battle updatedBattle = battle.accept(Instant.now());
 
         battleRepositoryPort.save(updatedBattle);
 
diff --git a/src/main/java/com/process/clash/application/compete/rival/battle/service/FindAllBattleInfoService.java b/src/main/java/com/process/clash/application/compete/rival/battle/service/FindAllBattleInfoService.java
index 1e1428b43..04e70834c 100644
--- a/src/main/java/com/process/clash/application/compete/rival/battle/service/FindAllBattleInfoService.java
+++ b/src/main/java/com/process/clash/application/compete/rival/battle/service/FindAllBattleInfoService.java
@@ -172,7 +172,7 @@ private BattleInfo toBattleInfo(
         );
 
         LocalDate expireDate = battle.endAt() != null
-                ? battle.endAt().atZone(battleZoneId).toLocalDate().minusDays(1)
+                ? battle.endAt().minusNanos(1).atZone(battleZoneId).toLocalDate()
                 : null;
 
         return BattleInfo.of(
diff --git a/src/main/java/com/process/clash/application/compete/rival/battle/service/FindDetailedBattleInfoService.java b/src/main/java/com/process/clash/application/compete/rival/battle/service/FindDetailedBattleInfoService.java
index c10188a4d..22a105b43 100644
--- a/src/main/java/com/process/clash/application/compete/rival/battle/service/FindDetailedBattleInfoService.java
+++ b/src/main/java/com/process/clash/application/compete/rival/battle/service/FindDetailedBattleInfoService.java
@@ -39,14 +39,14 @@ public FindDetailedBattleInfoData.Result execute(FindDetailedBattleInfoData.Comm
                 ? battle.startedAt().atZone(battleZoneId).toLocalDate()
                 : LocalDate.now(battleZoneId);
 
-        // exp 계산 기간 종료일: DONE이면 마지막 활동일(endAt 자정의 전날), 진행 중이면 오늘까지
+        // exp 계산 기간 종료일: DONE이면 endAt 기준 날짜, 진행 중이면 오늘까지
         LocalDate endDate = battle.battleStatus().equals(BattleStatus.DONE) && battle.endAt() != null
-                ? battle.endAt().atZone(battleZoneId).toLocalDate().minusDays(1)
+                ? battle.endAt().minusNanos(1).atZone(battleZoneId).toLocalDate()
                 : LocalDate.now(battleZoneId);
 
-        // 클라이언트 표시용 종료 예정일: endAt 자정의 전날 = 마지막 활동일
+        // 클라이언트 표시용 종료 예정일: endAt 기준 날짜 (마지막 활동일)
         LocalDate expireDate = battle.endAt() != null
-                ? battle.endAt().atZone(battleZoneId).toLocalDate().minusDays(1)
+                ? battle.endAt().minusNanos(1).atZone(battleZoneId).toLocalDate()
                 : null;
 
         // 상대방이 탈퇴해 라이벌이 삭제된 경우 상대 정보 없이 반환
diff --git a/src/main/java/com/process/clash/domain/rival/battle/entity/Battle.java b/src/main/java/com/process/clash/domain/rival/battle/entity/Battle.java
index 7d9cdb84f..08f7aa121 100644
--- a/src/main/java/com/process/clash/domain/rival/battle/entity/Battle.java
+++ b/src/main/java/com/process/clash/domain/rival/battle/entity/Battle.java
@@ -3,8 +3,7 @@
 import com.process.clash.domain.rival.battle.enums.BattleStatus;
 
 import java.time.Instant;
-import java.time.LocalDate;
-import java.time.ZoneId;
+import java.time.temporal.ChronoUnit;
 
 public record Battle(
         Long id,
@@ -35,9 +34,8 @@ public static Battle createDefault(int duration, Long rivalId, Long applicantId)
         );
     }
 
-    public Battle accept(Instant now, ZoneId zoneId) {
-        LocalDate acceptanceDateKST = now.atZone(zoneId).toLocalDate();
-        Instant endAt = acceptanceDateKST.plusDays(this.duration).atStartOfDay(zoneId).toInstant();
+    public Battle accept(Instant now) {
+        Instant endAt = now.plus(this.duration, ChronoUnit.DAYS);
 
         return new Battle(
                 this.id,
diff --git a/src/test/java/com/process/clash/application/compete/rival/battle/service/AcceptBattleServiceTest.java b/src/test/java/com/process/clash/application/compete/rival/battle/service/AcceptBattleServiceTest.java
index 884e3392f..4224ad3e5 100644
--- a/src/test/java/com/process/clash/application/compete/rival/battle/service/AcceptBattleServiceTest.java
+++ b/src/test/java/com/process/clash/application/compete/rival/battle/service/AcceptBattleServiceTest.java
@@ -19,7 +19,6 @@
 import org.mockito.junit.jupiter.MockitoExtension;
 
 import java.time.Instant;
-import java.time.ZoneId;
 import java.util.Collection;
 import java.util.Optional;
 
@@ -47,8 +46,6 @@ class AcceptBattleServiceTest {
 
     private AcceptBattleService acceptBattleService;
 
-    private static final ZoneId BATTLE_ZONE_ID = ZoneId.of("Asia/Seoul");
-
     @BeforeEach
     void setUp() {
         acceptBattleService = new AcceptBattleService(
@@ -56,8 +53,7 @@ void setUp() {
             modifyBattlePolicy,
             rivalRepositoryPort,
             userNoticeRepositoryPort,
-            competeRefetchNotifier,
-            BATTLE_ZONE_ID
+            competeRefetchNotifier
         );
     }
 
@@ -71,7 +67,7 @@ void execute_notifiesBothUsersOnAccept() {
         Battle battle = pendingBattle(battleId, rivalId, opponentId);
 
         when(battleRepositoryPort.findById(battleId)).thenReturn(Optional.of(battle));
-        when(battleRepositoryPort.save(any(Battle.class))).thenReturn(battle.accept(Instant.now(), BATTLE_ZONE_ID));
+        when(battleRepositoryPort.save(any(Battle.class))).thenReturn(battle.accept(Instant.now()));
         when(rivalRepositoryPort.findOpponentIdByIdAndUserId(rivalId, actor.id())).thenReturn(opponentId);
         when(userNoticeRepositoryPort.save(any(UserNotice.class))).thenAnswer(invocation -> invocation.getArgument(0));
 
@@ -96,7 +92,7 @@ void execute_savesOneNoticeOnAccept() {
         Battle battle = pendingBattle(battleId, rivalId, opponentId);
 
         when(battleRepositoryPort.findById(battleId)).thenReturn(Optional.of(battle));
-        when(battleRepositoryPort.save(any(Battle.class))).thenReturn(battle.accept(Instant.now(), BATTLE_ZONE_ID));
+        when(battleRepositoryPort.save(any(Battle.class))).thenReturn(battle.accept(Instant.now()));
         when(rivalRepositoryPort.findOpponentIdByIdAndUserId(rivalId, actor.id())).thenReturn(opponentId);
         when(userNoticeRepositoryPort.save(any(UserNotice.class))).thenAnswer(invocation -> invocation.getArgument(0));
 
@@ -115,7 +111,7 @@ void execute_softDeletesApplyBattleNoticeOnAccept() {
         Battle battle = pendingBattle(battleId, rivalId, opponentId);
 
         when(battleRepositoryPort.findById(battleId)).thenReturn(Optional.of(battle));
-        when(battleRepositoryPort.save(any(Battle.class))).thenReturn(battle.accept(Instant.now(), BATTLE_ZONE_ID));
+        when(battleRepositoryPort.save(any(Battle.class))).thenReturn(battle.accept(Instant.now()));
         when(rivalRepositoryPort.findOpponentIdByIdAndUserId(rivalId, actor.id())).thenReturn(opponentId);
         when(userNoticeRepositoryPort.save(any(UserNotice.class))).thenAnswer(invocation -> invocation.getArgument(0));
 
diff --git a/src/test/java/com/process/clash/application/compete/rival/battle/service/FindAllBattleInfoServiceTest.java b/src/test/java/com/process/clash/application/compete/rival/battle/service/FindAllBattleInfoServiceTest.java
index 68e30c5c3..e8016a8b7 100644
--- a/src/test/java/com/process/clash/application/compete/rival/battle/service/FindAllBattleInfoServiceTest.java
+++ b/src/test/java/com/process/clash/application/compete/rival/battle/service/FindAllBattleInfoServiceTest.java
@@ -190,7 +190,7 @@ void execute_returnsEnemyTierAsRankTier_whenRankTierIsNotNone() {
     }
 
     @Test
-    @DisplayName("IN_PROGRESS 배틀의 expireDate는 endAt을 KST 기준 LocalDate로 변환한 값이다")
+    @DisplayName("IN_PROGRESS 배틀의 expireDate는 endAt의 KST 날짜(마지막 활동일)이다")
     void execute_returnsExpireDateFromEndAt_whenInProgress() {
         Instant endAt = Instant.now().plus(6, ChronoUnit.DAYS);
         Battle battle = new Battle(BATTLE_ID, Instant.now(), Instant.now(),
@@ -205,7 +205,7 @@ void execute_returnsExpireDateFromEndAt_whenInProgress() {
 
         FindAllBattleInfoData.Result result = findAllBattleInfoService.execute(command());
 
-        LocalDate expectedDate = endAt.atZone(ZoneId.of("Asia/Seoul")).toLocalDate();
+        LocalDate expectedDate = endAt.minusNanos(1).atZone(ZoneId.of("Asia/Seoul")).toLocalDate();
         assertThat(result.battles().get(0).expireDate()).isEqualTo(expectedDate);
     }
 
diff --git a/src/test/java/com/process/clash/domain/rival/battle/entity/BattleTest.java b/src/test/java/com/process/clash/domain/rival/battle/entity/BattleTest.java
index 875976a48..f65e5f13a 100644
--- a/src/test/java/com/process/clash/domain/rival/battle/entity/BattleTest.java
+++ b/src/test/java/com/process/clash/domain/rival/battle/entity/BattleTest.java
@@ -5,8 +5,6 @@
 import org.junit.jupiter.api.Test;
 
 import java.time.Instant;
-import java.time.LocalDate;
-import java.time.ZoneId;
 import java.time.temporal.ChronoUnit;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -14,16 +12,14 @@
 class BattleTest {
 
     @Test
-    @DisplayName("승인 즉시 startedAt이 설정되고, endAt은 수락일(KST) + duration일의 자정 KST로 설정된다")
+    @DisplayName("승인 즉시 startedAt이 설정되고, endAt은 수락 시각으로부터 duration일(72시간 단위) 후로 설정된다")
     void accept_setsStartedAtAndReturnsInProgress() {
         Battle battle = pendingBattle(7);
         Instant now = Instant.now();
-        ZoneId zoneId = ZoneId.of("Asia/Seoul");
 
-        Battle result = battle.accept(now, zoneId);
+        Battle result = battle.accept(now);
 
-        LocalDate acceptanceDateKST = now.atZone(zoneId).toLocalDate();
-        Instant expectedEndAt = acceptanceDateKST.plusDays(7).atStartOfDay(zoneId).toInstant();
+        Instant expectedEndAt = now.plus(7, ChronoUnit.DAYS);
 
         assertThat(result.battleStatus()).isEqualTo(BattleStatus.IN_PROGRESS);
         assertThat(result.startedAt()).isEqualTo(now);