diff --git a/frontend/src/components/gantt/GanttChart.vue b/frontend/src/components/gantt/GanttChart.vue
index 23fab0cc3..f60c255ac 100644
--- a/frontend/src/components/gantt/GanttChart.vue
+++ b/frontend/src/components/gantt/GanttChart.vue
@@ -85,6 +85,7 @@ import GanttTimelineHeader from '@/components/gantt/GanttTimelineHeader.vue'
 import Loading from '@/components/misc/Loading.vue'
 
 import {MILLISECONDS_A_DAY} from '@/constants/date'
+import {roundToNaturalDayBoundary} from '@/helpers/time/roundToNaturalDayBoundary'
 
 const props = defineProps<{
 	isLoading: boolean,
@@ -149,16 +150,16 @@ const ganttBars = ref<GanttBarModel[][]>([])
 const ganttRows = ref<string[]>([])
 const cellsByRow = ref<Record<string, string[]>>({})
 
+function getRoundedDate(value: string | Date | undefined, fallback: Date, isStart: boolean) {
+	return roundToNaturalDayBoundary(value ? new Date(value) : new Date(fallback), isStart)
+}
+
 function transformTaskToGanttBar(t: ITask): GanttBarModel {
-	const startDate = t.startDate 
-		? new Date(t.startDate) 
-		: new Date(props.defaultTaskStartDate)
-	const endDate = t.endDate 
-		? new Date(t.endDate) 
-		: new Date(props.defaultTaskEndDate)
-	
+	const startDate = getRoundedDate(t.startDate, props.defaultTaskStartDate, true)
+	const endDate = getRoundedDate(t.endDate, props.defaultTaskEndDate, false)
+
 	const taskColor = getHexColor(t.hexColor)
-	
+
 	const bar = {
 		id: String(t.id),
 		start: startDate,
@@ -171,7 +172,7 @@ function transformTaskToGanttBar(t: ITask): GanttBarModel {
 			isDone: t.done,
 		},
 	}
-	
+
 	return bar
 }
 
@@ -181,22 +182,18 @@ watch(
 		const bars: GanttBarModel[] = []
 		const rows: string[] = []
 		const cells: Record<string, string[]> = {}
-		
+
 		const filteredTasks = Array.from(tasks.value.values()).filter(task => {
 			if (!filters.value.showTasksWithoutDates && (!task.startDate || !task.endDate)) {
 				return false
 			}
-			
-			const taskStart = task.startDate 
-				? new Date(task.startDate) 
-				: new Date(props.defaultTaskStartDate)
-			const taskEnd = task.endDate 
-				? new Date(task.endDate) 
-				: new Date(props.defaultTaskEndDate)
-			
+
+			const taskStart = getRoundedDate(task.startDate, props.defaultTaskStartDate, true)
+			const taskEnd = getRoundedDate(task.endDate, props.defaultTaskEndDate, false)
+
 			// Task is visible if it overlaps with the current date range
-			return taskStart <= dateToDate.value 
-				&& taskEnd >= dateFromDate.value
+			return taskStart <= dateToDate.value
+&& taskEnd >= dateFromDate.value
 		})
 		
 		filteredTasks.forEach((t, index) => {
@@ -225,8 +222,8 @@ watch(
 function updateGanttTask(id: string, newStart: Date, newEnd: Date) {
 	emit('update:task', {
 		id: Number(id),
-		startDate: dayjs(newStart).startOf('day').toDate(),
-		endDate: dayjs(newEnd).endOf('day').toDate(),
+		startDate: roundToNaturalDayBoundary(newStart, true),
+		endDate: roundToNaturalDayBoundary(newEnd),
 	})
 }
 
diff --git a/frontend/src/components/gantt/GanttRowBars.vue b/frontend/src/components/gantt/GanttRowBars.vue
index 7ae7f429e..9bc7e9111 100644
--- a/frontend/src/components/gantt/GanttRowBars.vue
+++ b/frontend/src/components/gantt/GanttRowBars.vue
@@ -104,6 +104,7 @@ import dayjs from 'dayjs'
 import type {GanttBarModel} from '@/composables/useGanttBar'
 import {getTextColor, LIGHT} from '@/helpers/color/getTextColor'
 import {MILLISECONDS_A_DAY} from '@/constants/date'
+import {roundToNaturalDayBoundary} from '@/helpers/time/roundToNaturalDayBoundary'
 
 import GanttBarPrimitive from './primitives/GanttBarPrimitive.vue'
 
@@ -152,7 +153,10 @@ function computeBarX(startDate: Date) {
 }
 
 function getDaysDifference(startDate: Date, endDate: Date): number {
-	return Math.floor((endDate.getTime() - startDate.getTime()) / MILLISECONDS_A_DAY)
+	return Math.ceil(
+		(roundToNaturalDayBoundary(endDate).getTime() - roundToNaturalDayBoundary(startDate, true).getTime()) /
+MILLISECONDS_A_DAY,
+	)
 }
 
 function computeBarWidth(bar: GanttBarModel) {
diff --git a/frontend/src/helpers/time/roundToNaturalDayBoundary.test.ts b/frontend/src/helpers/time/roundToNaturalDayBoundary.test.ts
new file mode 100644
index 000000000..b29850ada
--- /dev/null
+++ b/frontend/src/helpers/time/roundToNaturalDayBoundary.test.ts
@@ -0,0 +1,30 @@
+import {describe, it, expect} from 'vitest'
+
+import {roundToNaturalDayBoundary} from './roundToNaturalDayBoundary'
+import {MILLISECONDS_A_DAY} from '@/constants/date'
+
+describe('roundToNaturalDayBoundary', () => {
+	it('rounds start dates to start of day regardless of time', () => {
+		const morning = new Date('2024-01-01T08:00:00')
+		const afternoon = new Date('2024-01-01T13:00:00')
+		expect(roundToNaturalDayBoundary(morning, true)).toEqual(new Date('2024-01-01T00:00:00.000'))
+		expect(roundToNaturalDayBoundary(afternoon, true)).toEqual(new Date('2024-01-01T00:00:00.000'))
+	})
+
+	it('rounds end dates to natural boundaries', () => {
+		const morning = new Date('2024-01-01T08:00:00')
+		const afternoon = new Date('2024-01-01T13:00:00')
+		expect(roundToNaturalDayBoundary(morning)).toEqual(new Date('2024-01-01T00:00:00.000'))
+		expect(roundToNaturalDayBoundary(afternoon)).toEqual(new Date('2024-01-01T23:59:59.999'))
+	})
+
+	it('counts inclusive days for same-day evening end', () => {
+		const start = new Date('2024-01-01T15:00:00')
+		const end = new Date('2024-01-01T18:00:00')
+		const diff = Math.ceil(
+			(roundToNaturalDayBoundary(end).getTime() - roundToNaturalDayBoundary(start, true).getTime()) /
+				MILLISECONDS_A_DAY,
+		)
+		expect(diff).toBe(1)
+	})
+})
diff --git a/frontend/src/helpers/time/roundToNaturalDayBoundary.ts b/frontend/src/helpers/time/roundToNaturalDayBoundary.ts
new file mode 100644
index 000000000..8586914cb
--- /dev/null
+++ b/frontend/src/helpers/time/roundToNaturalDayBoundary.ts
@@ -0,0 +1,9 @@
+export function roundToNaturalDayBoundary(date: Date, isStart = false): Date {
+	const d = new Date(date)
+	if (isStart || d.getHours() < 12) {
+		d.setHours(0, 0, 0, 0)
+	} else {
+		d.setHours(23, 59, 59, 999)
+	}
+	return d
+}