diff --git a/frontend/src/views/DiaryView.vue b/frontend/src/views/DiaryView.vue
index 0dfb6267..377cd1dc 100644
--- a/frontend/src/views/DiaryView.vue
+++ b/frontend/src/views/DiaryView.vue
@@ -255,7 +255,7 @@
                                         </option>
                                     </select>
                                 </div>
-                                <div v-if="addNewItem || addNewTimeblock" class="plan-composer-field">
+                                <div v-if="addNewItem || addNewTimeblock || addNewGroupActivity" class="plan-composer-field">
                                     <label>{{ $t('diary.durationMinutes') }}</label>
                                     <div class="plan-composer-duration">
                                         <input type="text" v-model="newPlanItem.durationText" @input="calculateDuration" :placeholder="$t('diary.durationExampleLong')" />
@@ -2317,36 +2317,42 @@ export default {
             return minutes1 > minutes2;
         },
         calculateAllItemTimes() {
-            let currentTime = this.trainingStart;
-            let index = 0;
-            const trainingPlanLength = this.trainingPlan.length;
+            const initialTime = this.trainingStart || '00:00';
+            let globalCursor = initialTime;
+            const groupCursorMap = new Map();
+            const items = Array.isArray(this.trainingPlan) ? this.trainingPlan : [];
 
-            while (index < trainingPlanLength) {
-                const currentItem = this.trainingPlan[index];
+            for (const item of items) {
+                const hasGroup = Number.isFinite(Number(item?.groupId)) && Number(item.groupId) > 0;
+                const duration = Number.isFinite(Number(item?.duration)) ? Number(item.duration) : 0;
 
-                if (!currentItem.groupId) {
-                    currentItem.startTime = currentTime;
-                    currentItem.endTime = this.addDurationToTime(currentItem.startTime, currentItem.duration);
-                    currentTime = currentItem.endTime;
-                    index += 1;
-                } else {
-                    const groupActivities = [];
-                    while (index < trainingPlanLength && this.trainingPlan[index].groupId) {
-                        groupActivities.push(this.trainingPlan[index]);
-                        index += 1;
-                    }
-                    for (const item of groupActivities) {
-                        item.startTime = currentTime;
-                        item.endTime = this.addDurationToTime(item.startTime, item.duration);
-                    }
-                    let latestEndTime = currentTime;
-                    for (const item of groupActivities) {
-                        if (this.isTimeGreater(item.endTime, latestEndTime)) {
-                            latestEndTime = item.endTime;
+                if (!hasGroup) {
+                    let maxGroupCursor = globalCursor;
+                    for (const cursor of groupCursorMap.values()) {
+                        if (this.isTimeGreater(cursor, maxGroupCursor)) {
+                            maxGroupCursor = cursor;
                         }
                     }
-                    currentTime = latestEndTime;
+                    const startTime = this.isTimeGreater(maxGroupCursor, globalCursor) ? maxGroupCursor : globalCursor;
+                    item.startTime = startTime;
+                    item.endTime = this.addDurationToTime(startTime, duration);
+                    globalCursor = item.endTime;
+                    for (const [groupId, cursor] of groupCursorMap.entries()) {
+                        if (this.isTimeGreater(globalCursor, cursor)) {
+                            groupCursorMap.set(groupId, globalCursor);
+                        }
+                    }
+                    continue;
                 }
+
+                const groupId = Number(item.groupId);
+                if (!groupCursorMap.has(groupId)) {
+                    groupCursorMap.set(groupId, globalCursor);
+                }
+                const groupCursor = groupCursorMap.get(groupId) || globalCursor;
+                item.startTime = groupCursor;
+                item.endTime = this.addDurationToTime(groupCursor, duration);
+                groupCursorMap.set(groupId, item.endTime);
             }
         },