diff --git a/assets/js/dashboard/annotations/annotation-list-items.tsx b/assets/js/dashboard/annotations/annotation-list-items.tsx
index 03ebe3feb8e8..dc5a3c04437e 100644
--- a/assets/js/dashboard/annotations/annotation-list-items.tsx
+++ b/assets/js/dashboard/annotations/annotation-list-items.tsx
@@ -21,24 +21,31 @@ const VerticalBar = () => (
 )
 
 export const AnnotationItemRow = ({ children }: { children: ReactNode }) => (
-  <div className="group flex flex-row gap-x-2">
+  <div className="relative group flex flex-row gap-x-2">
     <VerticalBar />
     {children}
   </div>
 )
 
 export const AnnotationAuthorshipLine = ({
-  annotation
+  annotation,
+  showDateLabel
 }: {
   annotation: Annotation
+  showDateLabel: boolean
 }) => (
-  <div className="flex items-baseline text-xs text-gray-300 pr-8">
+  <div
+    data-testid="annotation-attribution"
+    className="flex items-baseline text-xs text-gray-300 pr-8"
+  >
     <span className="truncate min-w-0">
       {getAnnotationAuthorship(annotation)}
     </span>
-    <span className="whitespace-nowrap shrink-0">
-      {` • ${getAttributionDateLabel(annotation)}`}
-    </span>
+    {showDateLabel && (
+      <span className="whitespace-nowrap shrink-0">
+        &nbsp;{`• ${getAttributionDateLabel(annotation)}`}
+      </span>
+    )}
   </div>
 )
 
diff --git a/assets/js/dashboard/annotations/annotations.test.ts b/assets/js/dashboard/annotations/annotations.test.ts
index 0b36eb0b2a1b..086c6f08e16d 100644
--- a/assets/js/dashboard/annotations/annotations.test.ts
+++ b/assets/js/dashboard/annotations/annotations.test.ts
@@ -5,8 +5,9 @@ import {
   canEditAnnotation,
   getAnnotationAuthorship,
   getAnnotationGranularity,
-  getAnnotationTimeLabel,
-  groupAnnotationsByTimeLabel
+  getAnnotationDatetimeGroup,
+  groupAnnotationsByDatetime,
+  allAnnotationsAreFromThisExactDatetime
 } from './annotations'
 import { Interval } from '../stats/graph/intervals'
 import { Role, UserContextValue } from '../user-context'
@@ -199,7 +200,7 @@ describe(`${getAnnotationGranularity.name}`, () => {
   })
 })
 
-describe(`${getAnnotationTimeLabel.name}`, () => {
+describe(`${getAnnotationDatetimeGroup.name}`, () => {
   // 2025-02-26 is a Wednesday
   const dateAnnotation = {
     datetime: '2025-02-26',
@@ -215,7 +216,9 @@ describe(`${getAnnotationTimeLabel.name}`, () => {
   ])(
     `date-granularity annotation on ${dateAnnotation.datetime} bucketed to %s yields %s`,
     (interval, expected) => {
-      expect(getAnnotationTimeLabel(dateAnnotation, interval)).toBe(expected)
+      expect(getAnnotationDatetimeGroup(dateAnnotation, interval)).toBe(
+        expected
+      )
     }
   )
 
@@ -227,17 +230,19 @@ describe(`${getAnnotationTimeLabel.name}`, () => {
     [Interval.month, '2025-02-01'],
     [Interval.week, '2025-02-24'],
     [Interval.day, '2025-02-26'],
-    [Interval.hour, '2025-02-26 10:00:00'],
-    [Interval.minute, '2025-02-26 10:30:00']
+    [Interval.hour, '2025-02-26T10:00:00'],
+    [Interval.minute, '2025-02-26T10:30:00']
   ])(
     `minute granularity annotation with datetime ${minuteAnnotation.datetime} bucketed to %s yields %s`,
     (interval, expected) => {
-      expect(getAnnotationTimeLabel(minuteAnnotation, interval)).toBe(expected)
+      expect(getAnnotationDatetimeGroup(minuteAnnotation, interval)).toBe(
+        expected
+      )
     }
   )
 })
 
-describe(`${groupAnnotationsByTimeLabel.name}`, () => {
+describe(`${groupAnnotationsByDatetime.name}`, () => {
   const dateGranularity = AnnotationGranularity.date
   const annotations = [
     { id: 1, datetime: '2025-02-24 00:00:00', granularity: dateGranularity }, // Mon
@@ -246,7 +251,7 @@ describe(`${groupAnnotationsByTimeLabel.name}`, () => {
   ]
 
   it('groups annotations by day when the interval is day', () => {
-    const grouped = groupAnnotationsByTimeLabel(annotations, Interval.day)
+    const grouped = groupAnnotationsByDatetime(annotations, Interval.day)
 
     expect(Object.keys(grouped).sort()).toEqual([
       '2025-02-24',
@@ -259,7 +264,7 @@ describe(`${groupAnnotationsByTimeLabel.name}`, () => {
   })
 
   it('collapses annotations from the same week into one bucket', () => {
-    const grouped = groupAnnotationsByTimeLabel(annotations, Interval.week)
+    const grouped = groupAnnotationsByDatetime(annotations, Interval.week)
 
     expect(Object.keys(grouped).sort()).toEqual(['2025-02-24', '2025-03-03'])
     expect(grouped['2025-02-24']!.map((a) => a.id)).toEqual([1, 2])
@@ -267,7 +272,7 @@ describe(`${groupAnnotationsByTimeLabel.name}`, () => {
   })
 
   it('collapses annotations from the same month into one bucket', () => {
-    const grouped = groupAnnotationsByTimeLabel(annotations, Interval.month)
+    const grouped = groupAnnotationsByDatetime(annotations, Interval.month)
 
     expect(Object.keys(grouped).sort()).toEqual(['2025-02-01', '2025-03-01'])
     expect(grouped['2025-02-01']!.map((a) => a.id)).toEqual([1, 2])
@@ -281,17 +286,58 @@ describe(`${groupAnnotationsByTimeLabel.name}`, () => {
       { id: 12, datetime: '2025-02-26 10:00:00', granularity: dateGranularity }
     ]
 
-    const grouped = groupAnnotationsByTimeLabel(sameDay, Interval.day)
+    const grouped = groupAnnotationsByDatetime(sameDay, Interval.day)
 
     expect(grouped['2025-02-26']!.map((a) => a.id)).toEqual([10, 11, 12])
   })
 
   it('returns an empty object when there are no annotations', () => {
     expect(
-      groupAnnotationsByTimeLabel(
+      groupAnnotationsByDatetime(
         [] as { datetime: string; granularity: AnnotationGranularity }[],
         Interval.day
       )
     ).toEqual({})
   })
 })
+
+describe(`${allAnnotationsAreFromThisExactDatetime.name}`, () => {
+  it('returns true when every annotation shares the given datetime', () => {
+    expect(
+      allAnnotationsAreFromThisExactDatetime(
+        [
+          { datetime: '2025-02-26T10:30:00' },
+          { datetime: '2025-02-26T10:30:00' }
+        ],
+        '2025-02-26T10:30:00'
+      )
+    ).toBe(true)
+  })
+
+  it('returns true when every annotation shares the given date', () => {
+    expect(
+      allAnnotationsAreFromThisExactDatetime(
+        [{ datetime: '2026-07-20' }, { datetime: '2026-07-20' }],
+        '2026-07-20'
+      )
+    ).toBe(true)
+  })
+
+  it('returns false when some annotation has a different datetime', () => {
+    expect(
+      allAnnotationsAreFromThisExactDatetime(
+        [
+          { datetime: '2025-02-26T10:30:00' },
+          { datetime: '2025-02-26T10:31:00' }
+        ],
+        '2025-02-26T10:30:00'
+      )
+    ).toBe(false)
+  })
+
+  it('returns true when there are no annotations', () => {
+    expect(
+      allAnnotationsAreFromThisExactDatetime([], '2025-02-26T10:30:00')
+    ).toBe(true)
+  })
+})
diff --git a/assets/js/dashboard/annotations/annotations.ts b/assets/js/dashboard/annotations/annotations.ts
index 4ba1faf96606..b848ba056285 100644
--- a/assets/js/dashboard/annotations/annotations.ts
+++ b/assets/js/dashboard/annotations/annotations.ts
@@ -37,9 +37,9 @@ export type Annotation = {
   note: string
 
   id: number
-  /** datetime in site timezone, example 2025-02-26 10:00:00 */
+  /** datetime in site timezone, example 2025-02-26T10:00:00 */
   inserted_at: string
-  /** datetime in site timezone, example 2025-02-26 10:00:00 */
+  /** datetime in site timezone, example 2025-02-26T10:00:00 */
   updated_at: string
 } & AnnotationOwnership
 
@@ -139,7 +139,7 @@ export function canAddAnnotation({
   }
 }
 
-export const getAnnotationTimeLabel = (
+export const getAnnotationDatetimeGroup = (
   annotation: Pick<Annotation, 'datetime' | 'granularity'>,
   interval: Interval
 ): string => {
@@ -175,24 +175,24 @@ export const getAnnotationTimeLabel = (
         case Interval.hour: {
           const [dateYYYYMMDD, timeHHMMSS] = annotation.datetime.split('T')
           // floors time to hour
-          return `${dateYYYYMMDD} ${timeHHMMSS.substring(0, 'HH'.length)}:00:00`
+          return `${dateYYYYMMDD}T${timeHHMMSS.substring(0, 'HH'.length)}:00:00`
         }
         case Interval.minute:
-          return annotation.datetime.split('T').join(' ')
+          return annotation.datetime
       }
     }
   }
 }
 
-export const groupAnnotationsByTimeLabel = <
+export const groupAnnotationsByDatetime = <
   T extends Pick<Annotation, 'datetime' | 'granularity'>
 >(
   annotations: T[],
   interval: Interval
 ): Record<string, T[] | undefined> => {
   return annotations.reduce<Record<string, T[]>>((acc, annotation) => {
-    const timeLabel = getAnnotationTimeLabel(annotation, interval)
-    return { ...acc, [timeLabel]: [...(acc[timeLabel] ?? []), annotation] }
+    const label = getAnnotationDatetimeGroup(annotation, interval)
+    return { ...acc, [label]: [...(acc[label] ?? []), annotation] }
   }, {})
 }
 
@@ -210,6 +210,11 @@ export const getAnnotationGranularity = (
   }
 }
 
+export const allAnnotationsAreFromThisExactDatetime = (
+  annotations: Pick<Annotation, 'datetime'>[],
+  datetime: string
+): boolean => annotations.every((a) => a.datetime === datetime)
+
 export const getApiFormattedPayload = ({
   granularity,
   datetime,
diff --git a/assets/js/dashboard/annotations/hover-annotations-list.tsx b/assets/js/dashboard/annotations/hover-annotations-list.tsx
index 2acc232c6103..01522197d943 100644
--- a/assets/js/dashboard/annotations/hover-annotations-list.tsx
+++ b/assets/js/dashboard/annotations/hover-annotations-list.tsx
@@ -1,5 +1,8 @@
 import React from 'react'
-import { Annotation } from './annotations'
+import {
+  Annotation,
+  allAnnotationsAreFromThisExactDatetime
+} from './annotations'
 import {
   AnnotationAuthorshipLine,
   AnnotationItemRow,
@@ -10,20 +13,29 @@ import {
 const MAX_PREVIEW = 2
 
 export const HoverAnnotationsList = ({
+  annotationDatetime,
   annotations
 }: {
+  annotationDatetime: string
   annotations: Annotation[]
 }) => {
   const preview = annotations.slice(0, MAX_PREVIEW)
   const extra = annotations.length - MAX_PREVIEW
+  const showDateLabel = !allAnnotationsAreFromThisExactDatetime(
+    preview,
+    annotationDatetime
+  )
 
   return (
     <>
       <AnnotationsListContainer>
         {preview.map((annotation) => (
           <AnnotationItemRow key={annotation.id}>
-            <div className="relative flex flex-col gap-y-px w-full max-w-64">
-              <AnnotationAuthorshipLine annotation={annotation} />
+            <div className="flex flex-col gap-y-px w-full max-w-64">
+              <AnnotationAuthorshipLine
+                annotation={annotation}
+                showDateLabel={showDateLabel}
+              />
               <AnnotationNote note={annotation.note} clamp />
             </div>
           </AnnotationItemRow>
diff --git a/assets/js/dashboard/annotations/interactive-annotations-list.tsx b/assets/js/dashboard/annotations/interactive-annotations-list.tsx
index 987007df4d45..fd01220e74df 100644
--- a/assets/js/dashboard/annotations/interactive-annotations-list.tsx
+++ b/assets/js/dashboard/annotations/interactive-annotations-list.tsx
@@ -1,5 +1,9 @@
 import React, { ReactNode } from 'react'
-import { Annotation, canEditAnnotation } from './annotations'
+import {
+  Annotation,
+  allAnnotationsAreFromThisExactDatetime,
+  canEditAnnotation
+} from './annotations'
 import {
   AnnotationAuthorshipLine,
   AnnotationItemRow,
@@ -17,10 +21,12 @@ const ScrollableArea = (props: { children: ReactNode }) => (
 )
 
 export const InteractiveAnnotationsList = ({
+  annotationDatetime,
   annotations,
   isTouchDevice,
   closeTooltip
 }: {
+  annotationDatetime: string
   annotations: Annotation[]
   isTouchDevice: boolean
   closeTooltip: () => void
@@ -31,6 +37,10 @@ export const InteractiveAnnotationsList = ({
     closeTooltip()
     setModal({ type: 'update-annotation', annotation })
   }
+  const showDateLabel = !allAnnotationsAreFromThisExactDatetime(
+    annotations,
+    annotationDatetime
+  )
 
   return (
     <ScrollableArea>
@@ -39,7 +49,10 @@ export const InteractiveAnnotationsList = ({
           const editable = canEditAnnotation({ type: annotation.type, user })
           const content = (
             <>
-              <AnnotationAuthorshipLine annotation={annotation} />
+              <AnnotationAuthorshipLine
+                annotation={annotation}
+                showDateLabel={showDateLabel}
+              />
               <AnnotationNote note={annotation.note} />
               {editable && !isTouchDevice && (
                 <button
@@ -56,13 +69,13 @@ export const InteractiveAnnotationsList = ({
             <AnnotationItemRow key={annotation.id}>
               {editable && isTouchDevice ? (
                 <button
-                  className="relative flex flex-col gap-y-px w-full max-w-64 text-left focus:outline-none"
+                  className="flex flex-col gap-y-px w-full max-w-64 text-left focus:outline-none"
                   onClick={() => openEdit(annotation)}
                 >
                   {content}
                 </button>
               ) : (
-                <div className="relative flex flex-col gap-y-px w-full max-w-64">
+                <div className="flex flex-col gap-y-px w-full max-w-64">
                   {content}
                 </div>
               )}
diff --git a/assets/js/dashboard/stats/graph/main-graph-data.test.ts b/assets/js/dashboard/stats/graph/main-graph-data.test.ts
index 1f81532eb644..6f4620bf7130 100644
--- a/assets/js/dashboard/stats/graph/main-graph-data.test.ts
+++ b/assets/js/dashboard/stats/graph/main-graph-data.test.ts
@@ -1,7 +1,8 @@
 import {
   getChangeInPercentagePoints,
   getRelativeChange,
-  getLineSegments
+  getLineSegments,
+  normalizeGraphTimeLabel
 } from './main-graph-data'
 
 describe(`${getChangeInPercentagePoints.name}`, () => {
@@ -109,3 +110,15 @@ describe(`${getLineSegments.name}`, () => {
     ])
   })
 })
+
+describe(`${normalizeGraphTimeLabel.name}`, () => {
+  it('replaces the space separator with "T" for datetime labels', () => {
+    expect(normalizeGraphTimeLabel('2026-10-10 10:00:15')).toBe(
+      '2026-10-10T10:00:15'
+    )
+  })
+
+  it('leaves date-only labels unchanged', () => {
+    expect(normalizeGraphTimeLabel('2026-10-10')).toBe('2026-10-10')
+  })
+})
diff --git a/assets/js/dashboard/stats/graph/main-graph-data.ts b/assets/js/dashboard/stats/graph/main-graph-data.ts
index 3f34a9f73ab8..153d3dfb8eb3 100644
--- a/assets/js/dashboard/stats/graph/main-graph-data.ts
+++ b/assets/js/dashboard/stats/graph/main-graph-data.ts
@@ -213,3 +213,11 @@ type SeriesValue =
       isCurrent: boolean
       timeLabel: string
     }
+
+/**
+ * This function normalizes graph time labels in the format "2026-10-10 10:00:15"
+ * to the standard format "2026-10-10T10:00:15". Passes YYYY-MM-DD format date strings
+ * like "2026-01-01" through unchanged.
+ */
+export const normalizeGraphTimeLabel = (timeLabel: string) =>
+  timeLabel.split(' ').join('T')
diff --git a/assets/js/dashboard/stats/graph/main-graph.tsx b/assets/js/dashboard/stats/graph/main-graph.tsx
index bf8708ca329d..b41de291628b 100644
--- a/assets/js/dashboard/stats/graph/main-graph.tsx
+++ b/assets/js/dashboard/stats/graph/main-graph.tsx
@@ -36,7 +36,8 @@ import {
   getRelativeChange,
   REVENUE_METRICS,
   getFirstAndLastTimeLabels,
-  MainGraphSeriesName
+  MainGraphSeriesName,
+  normalizeGraphTimeLabel
 } from './main-graph-data'
 import { Metric, getMetricLabel } from '../metrics'
 import { extractIntervalFromDimensions, Interval } from './intervals'
@@ -46,7 +47,7 @@ import {
   AnnotationType,
   canShowAddAnnotationButton,
   getAnnotationGranularity,
-  groupAnnotationsByTimeLabel
+  groupAnnotationsByDatetime
 } from '../../annotations/annotations'
 import { useUserContext } from '../../user-context'
 import { Button } from '../../components/button'
@@ -118,8 +119,8 @@ export const MainGraph = ({
   const interval = extractIntervalFromDimensions(data.query.dimensions)
   const isRealtime = data.extraContext.isRealtime
 
-  const annotationsByTimeLabel = useMemo(
-    () => groupAnnotationsByTimeLabel(annotations, interval),
+  const annotationsByDatetime = useMemo(
+    () => groupAnnotationsByDatetime(annotations, interval),
     [annotations, interval]
   )
 
@@ -304,13 +305,15 @@ export const MainGraph = ({
     () =>
       remappedData.map((datum) => {
         const annotationsOnDatum = datum.main.isDefined
-          ? (annotationsByTimeLabel[datum.main.timeLabel] ?? [])
+          ? (annotationsByDatetime[
+              normalizeGraphTimeLabel(datum.main.timeLabel)
+            ] ?? [])
           : []
         return {
           count: annotationsOnDatum.length
         }
       }),
-    [remappedData, annotationsByTimeLabel]
+    [remappedData, annotationsByDatetime]
   )
 
   const getFormattedValue = useCallback(
@@ -398,9 +401,13 @@ export const MainGraph = ({
 
   const annotationDatetime =
     selectedDatum && selectedDatum.main.isDefined
-      ? selectedDatum.main.timeLabel
+      ? normalizeGraphTimeLabel(selectedDatum.main.timeLabel)
       : null
 
+  const annotationsForSelectedBucket = annotationDatetime
+    ? annotationsByDatetime[annotationDatetime]
+    : undefined
+
   const zoomToPeriod = useCallback(
     (date: string) => {
       setTooltip(initialTooltipState)
@@ -507,11 +514,7 @@ export const MainGraph = ({
           {tooltip.persistent ? (
             <PersistentTooltipContents
               annotationDatetime={annotationDatetime}
-              annotations={
-                annotationDatetime
-                  ? annotationsByTimeLabel[annotationDatetime]
-                  : undefined
-              }
+              annotations={annotationsForSelectedBucket}
               isTouchDevice={isTouchDevice}
               interval={interval}
               zoomDate={zoomDate}
@@ -521,11 +524,8 @@ export const MainGraph = ({
             />
           ) : (
             <HoveredTooltipContents
-              annotations={
-                annotationDatetime
-                  ? annotationsByTimeLabel[annotationDatetime]
-                  : undefined
-              }
+              annotationDatetime={annotationDatetime}
+              annotations={annotationsForSelectedBucket}
               interval={interval}
               zoomDate={zoomDate}
               canAddAnnotation={canAddAnnotation}
@@ -538,6 +538,7 @@ export const MainGraph = ({
 }
 
 type TooltipContentsProps = {
+  annotationDatetime: string | null
   annotations: Annotation[] | undefined
   interval: Interval
   zoomDate: string | null
@@ -554,7 +555,6 @@ const PersistentTooltipContents = ({
   canAddAnnotation,
   closeTooltip
 }: {
-  annotationDatetime: string | null
   isTouchDevice: boolean
   onZoomToPeriod: (date: string) => void
   closeTooltip: () => void
@@ -564,8 +564,9 @@ const PersistentTooltipContents = ({
   const hasActions = !!zoomDate || (!!annotationDatetime && canAddAnnotation)
   return (
     <>
-      {!!annotations?.length && (
+      {!!annotationDatetime && !!annotations?.length && (
         <InteractiveAnnotationsList
+          annotationDatetime={annotationDatetime}
           annotations={annotations}
           isTouchDevice={isTouchDevice}
           closeTooltip={closeTooltip}
@@ -606,6 +607,7 @@ const PersistentTooltipContents = ({
 }
 
 const HoveredTooltipContents = ({
+  annotationDatetime,
   annotations,
   interval,
   zoomDate,
@@ -613,8 +615,11 @@ const HoveredTooltipContents = ({
 }: TooltipContentsProps) => {
   return (
     <>
-      {!!annotations?.length && (
-        <HoverAnnotationsList annotations={annotations} />
+      {!!annotationDatetime && !!annotations?.length && (
+        <HoverAnnotationsList
+          annotations={annotations}
+          annotationDatetime={annotationDatetime}
+        />
       )}
       <hr className="border-gray-600 dark:border-gray-800 my-1" />
       <div className="flex flex-col gap-y-0.5">
diff --git a/e2e/tests/dashboard/annotations.spec.ts b/e2e/tests/dashboard/annotations.spec.ts
index aa15273f25d5..bf6f2fd3ac67 100644
--- a/e2e/tests/dashboard/annotations.spec.ts
+++ b/e2e/tests/dashboard/annotations.spec.ts
@@ -65,6 +65,10 @@ test('user can create annotations across granularities, edit, and delete them',
 
   const personalNoteAttribution = 'Personal note • 29 Jun 10:00'
   const siteNoteAttribution = `${user.name} • 29 Jun`
+  // when all annotations in a bucket are exactly for the date (& time) of the bucket
+  // the attribution is shortened because it's unambiguous
+  const siteNoteAttributionUnambiguous = `${user.name}`
+  const personalNoteAttributionUnambiguous = 'Personal note'
 
   // Asserts the tooltip's annotation list matches provided list exactly
   const expectTooltipAnnotations = async (
@@ -75,7 +79,9 @@ test('user can create annotations across granularities, edit, and delete them',
     for (const [i, { note, attribution }] of expected.entries()) {
       const row = rows.nth(i)
       await expect(row).toContainText(note)
-      await expect(row.getByText(attribution, { exact: true })).toBeVisible()
+      await expect(row.getByTestId('annotation-attribution')).toHaveText(
+        attribution
+      )
     }
   }
 
@@ -124,7 +130,7 @@ test('user can create annotations across granularities, edit, and delete them',
   await test.step('right-click Monday and zoom in via "View day"', async () => {
     await mondayIn7d.hover()
     await expectTooltipAnnotations([
-      { note: siteNoteText, attribution: siteNoteAttribution }
+      { note: siteNoteText, attribution: siteNoteAttributionUnambiguous }
     ])
 
     await mondayIn7d.click({ button: 'right' })
@@ -164,6 +170,18 @@ test('user can create annotations across granularities, edit, and delete them',
     ).toBeVisible()
   })
 
+  await test.step('in day view, hovering the 10:00 bucket with the single note omits the duplicative datetime label from the note', async () => {
+    const hour10InDay = page.getByTestId('graph-dot-series-1-bucket-10')
+    await hour10InDay.hover()
+    await expect(tooltip).toBeVisible()
+    await expectTooltipAnnotations([
+      {
+        note: personalNoteText,
+        attribution: personalNoteAttributionUnambiguous
+      }
+    ])
+  })
+
   await test.step('zoom back out to 7d, both annotations are visible on Monday', async () => {
     await page.goBack({ waitUntil: 'commit' })
     await setGraphInterval('Days')
@@ -239,7 +257,7 @@ test('user can create annotations across granularities, edit, and delete them',
     await expect(deleteModalHeading).toBeHidden()
     await mondayIn7d.hover()
     await expectTooltipAnnotations([
-      { note: siteNoteText, attribution: siteNoteAttribution }
+      { note: siteNoteText, attribution: siteNoteAttributionUnambiguous }
     ])
     await expect(mondayMarker7d).toBeVisible()
   })
diff --git a/test/plausible_web/controllers/api/internal_controller/annotations_controller_test.exs b/test/plausible_web/controllers/api/internal_controller/annotations_controller_test.exs
index 0f383668d4d5..047affef7492 100644
--- a/test/plausible_web/controllers/api/internal_controller/annotations_controller_test.exs
+++ b/test/plausible_web/controllers/api/internal_controller/annotations_controller_test.exs
@@ -495,26 +495,28 @@ defmodule PlausibleWeb.Api.Internal.AnnotationsControllerTest do
       assert response["datetime"] == "2026-01-04T14:32:00"
     end
 
-    test "converts naive local time to UTC and returns it back as local time",
-         %{conn: conn, user: user} do
-      # America/New_York is UTC-5 in January; input and output should be the same
-      # local time (round-trip), while the DB stores the UTC equivalent.
-      site = new_site(owner: user, timezone: "America/New_York")
+    for dt <- ["2026-01-04T14:30:00", "2026-01-04 14:30:00"] do
+      test "converts naive local time #{dt} to UTC and returns it back as local time",
+           %{conn: conn, user: user} do
+        # America/New_York is UTC-5 in January; input and output should be the same
+        # local time (round-trip), while the DB stores the UTC equivalent.
+        site = new_site(owner: user, timezone: "America/New_York")
 
-      response =
-        post(conn, "/api/#{site.domain}/annotations", %{
-          "note" => "feature released",
-          "type" => "personal",
-          "granularity" => "minute",
-          "datetime" => "2026-01-04T14:30:00"
-        })
-        |> json_response(200)
+        response =
+          post(conn, "/api/#{site.domain}/annotations", %{
+            "note" => "feature released",
+            "type" => "personal",
+            "granularity" => "minute",
+            "datetime" => unquote(dt)
+          })
+          |> json_response(200)
 
-      assert response["datetime"] == "2026-01-04T14:30:00"
+        assert response["datetime"] == "2026-01-04T14:30:00"
 
-      reloaded = Plausible.Repo.get!(Plausible.Annotations.Annotation, response["id"])
-      assert reloaded.granularity == :minute
-      assert reloaded.datetime == ~U[2026-01-04 19:30:00Z]
+        reloaded = Plausible.Repo.get!(Plausible.Annotations.Annotation, response["id"])
+        assert reloaded.granularity == :minute
+        assert reloaded.datetime == ~U[2026-01-04 19:30:00Z]
+      end
     end
 
     test "UTC datetime string is stored as UTC and returned as site local time",