diff --git a/src/main/java/org/ntlab/citrusserver/repositories/ScheduleManager.java b/src/main/java/org/ntlab/citrusserver/repositories/ScheduleManager.java
index 310befd..0a9a0c9 100644
--- a/src/main/java/org/ntlab/citrusserver/repositories/ScheduleManager.java
+++ b/src/main/java/org/ntlab/citrusserver/repositories/ScheduleManager.java
@@ -9,6 +9,8 @@
 import java.text.ParseException;
 import java.text.SimpleDateFormat;
 import java.time.LocalDate;
+import java.time.LocalDateTime;
+import java.time.format.DateTimeFormatter;
 import java.time.temporal.ChronoUnit;
 import java.util.*;
 import java.util.concurrent.ScheduledExecutorService;
@@ -168,45 +170,35 @@
             schedules.get(accountId).get(year).put(month, new HashMap<>());
         }
 
-        SimpleDateFormat sdf = new SimpleDateFormat("yyyy/MM/dd HH:mm");
-
-        Schedule lastCreatedSchedule = null;
-        System.out.println(day);
+        DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy/MM/dd HH:mm");
 
         int newScheduleId = nextScheduleId.computeIfAbsent(accountId, k -> 0);
-        int groupId = newScheduleId;
 
-        try {
-            Date startDate = sdf.parse(startTime);
-            Date endDate = sdf.parse(endTime);
+        LocalDateTime startDateTime = LocalDateTime.parse(startTime, formatter);
+        LocalDateTime endDateTime = LocalDateTime.parse(endTime, formatter);
 
-            Calendar scheduleCalendar = Calendar.getInstance();
-            scheduleCalendar.setTime(startDate);
+        LocalDate startDate = startDateTime.toLocalDate();
+        LocalDate endDate = endDateTime.toLocalDate();
 
-            while (!scheduleCalendar.getTime().after(endDate)) {
-                int currentYear = scheduleCalendar.get(Calendar.YEAR);
-                int currentMonth = scheduleCalendar.get(Calendar.MONTH) + 1; // +1で調整
-                int currentDay = scheduleCalendar.get(Calendar.DAY_OF_MONTH);
-
-                // スケジュールの初期化
-                schedules.get(accountId).computeIfAbsent(currentYear, k -> new HashMap<>()).computeIfAbsent(currentMonth, k -> new HashMap<>())
-                        .computeIfAbsent(currentDay, k -> new HashMap<>());
+        Schedule res = null;
+        long days = ChronoUnit.DAYS.between(startDate, endDate) + 1;
 
 
-                Schedule newSchedule = new Schedule(title, startTime, endTime, bookId, groupId);
-                schedules.get(accountId).get(currentYear).get(currentMonth).get(currentDay).put(groupId, newSchedule);
-                nextScheduleId.put(accountId, newScheduleId);
-                lastCreatedSchedule = newSchedule;
-
-                // 次の日に進む
-                scheduleCalendar.add(Calendar.DAY_OF_MONTH, 1);
-            }
-        } catch (ParseException e) {
-            e.printStackTrace(); // より具体的なエラーハンドリングを検討
-            return null;
+        for(long i = 0; i < days; i++) {
+            int curYear = startDate.getYear();
+            int curMonth = startDate.getMonthValue();
+            int curDay = startDate.getDayOfMonth();
+            schedules.get(accountId).computeIfAbsent(curYear, k -> new HashMap<>()).computeIfAbsent(curMonth, k -> new HashMap<>())
+                    .computeIfAbsent(curDay, k -> new HashMap<>());
+            Schedule newSchedule = new Schedule(title, startTime, endTime, bookId, newScheduleId);
+            schedules.get(accountId).get(curYear).get(curMonth).get(curDay).put(newScheduleId, newSchedule);
+            nextScheduleId.put(accountId, newScheduleId);
+            startDate = startDate.plusDays(1);
+            nextScheduleId.put(accountId, newScheduleId + 1);
+            res = newSchedule;
         }
-        nextScheduleId.put(accountId, newScheduleId + 1);
-        return lastCreatedSchedule;
+
+        return res;
     }
 
     public Schedule getScheduleId(String accountId, int year, int month, int day, int scheduleId, Integer bookId) {