diff --git a/CHANGELOG.md b/CHANGELOG.md
index 64382aacf..2a2a67ede 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 ### Changed
 - Use weekday abbreviations instead of single char weekday identifiers ([#103])
+- Converting all-day events to timed events now respects default start time and duration ([#917])
 
 ### Fixed
 - Fixed crashes and freezing affecting some devices ([#889])
@@ -195,6 +196,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 [#826]: https://github.com/FossifyOrg/Calendar/issues/826
 [#870]: https://github.com/FossifyOrg/Calendar/issues/870
 [#889]: https://github.com/FossifyOrg/Calendar/issues/889
+[#917]: https://github.com/FossifyOrg/Calendar/issues/917
 
 [Unreleased]: https://github.com/FossifyOrg/Calendar/compare/1.8.1...HEAD
 [1.8.1]: https://github.com/FossifyOrg/Calendar/compare/1.8.0...1.8.1
diff --git a/app/src/main/kotlin/org/fossify/calendar/activities/EventActivity.kt b/app/src/main/kotlin/org/fossify/calendar/activities/EventActivity.kt
index 7f50687c6..e48f3bb6d 100644
--- a/app/src/main/kotlin/org/fossify/calendar/activities/EventActivity.kt
+++ b/app/src/main/kotlin/org/fossify/calendar/activities/EventActivity.kt
@@ -46,6 +46,7 @@ import org.fossify.calendar.extensions.config
 import org.fossify.calendar.extensions.eventTypesDB
 import org.fossify.calendar.extensions.eventsDB
 import org.fossify.calendar.extensions.eventsHelper
+import org.fossify.calendar.extensions.getNewEventTimestampFromCode
 import org.fossify.calendar.extensions.getRepetitionText
 import org.fossify.calendar.extensions.getShortDaysFromBitmask
 import org.fossify.calendar.extensions.isXMonthlyRepetition
@@ -205,6 +206,7 @@ class EventActivity : SimpleActivity() {
     private var mOriginalEndTS = 0L
     private var mIsNewEvent = true
     private var mEventColor = 0
+    private var mConvertedFromOriginalAllDay = false
 
     private lateinit var mEventStartDateTime: DateTime
     private lateinit var mEventEndDateTime: DateTime
@@ -1542,6 +1544,33 @@ class EventActivity : SimpleActivity() {
 
     private fun toggleAllDay(isAllDay: Boolean) {
         hideKeyboard()
+
+        // when converting from all-day to timed for the first time,
+        // set default start time and duration to avoid spanning into next day
+        if (!isAllDay && mEvent.getIsAllDay() && !mConvertedFromOriginalAllDay) {
+            val defaultStartTS = getNewEventTimestampFromCode(Formatter.getDayCodeFromDateTime(mEventStartDateTime))
+            val defaultStartTime = Formatter.getDateTimeFromTS(defaultStartTS)
+            val defaultDurationMinutes = config.defaultDuration
+            val endTime = defaultStartTime.plusMinutes(defaultDurationMinutes)
+
+            mEventStartDateTime = mEventStartDateTime.withTime(
+                defaultStartTime.hourOfDay,
+                defaultStartTime.minuteOfHour,
+                0,
+                0
+            )
+            mEventEndDateTime = mEventEndDateTime.withTime(
+                endTime.hourOfDay,
+                endTime.minuteOfHour,
+                0,
+                0
+            )
+
+            mConvertedFromOriginalAllDay = true
+            updateStartTexts()
+            updateEndTexts()
+        }
+
         mIsAllDayEvent = isAllDay
         binding.eventStartTime.beGoneIf(isAllDay)
         binding.eventEndTime.beGoneIf(isAllDay)
diff --git a/app/src/main/kotlin/org/fossify/calendar/activities/TaskActivity.kt b/app/src/main/kotlin/org/fossify/calendar/activities/TaskActivity.kt
index cf3d77ec5..372db324e 100644
--- a/app/src/main/kotlin/org/fossify/calendar/activities/TaskActivity.kt
+++ b/app/src/main/kotlin/org/fossify/calendar/activities/TaskActivity.kt
@@ -46,6 +46,7 @@ class TaskActivity : SimpleActivity() {
     private var mLastSavePromptTS = 0L
     private var mIsNewTask = true
     private var mEventColor = 0
+    private var mConvertedFromOriginalAllDay = false
 
     private val binding by viewBinding(ActivityTaskBinding::inflate)
 
@@ -646,6 +647,24 @@ class TaskActivity : SimpleActivity() {
 
     private fun toggleAllDay(isChecked: Boolean) {
         hideKeyboard()
+
+        // One-time migration: when converting from all-day to timed for the first time,
+        // set default start time to avoid unexpected time values
+        if (!isChecked && mTask.getIsAllDay() && !mConvertedFromOriginalAllDay) {
+            val defaultStartTS = getNewEventTimestampFromCode(Formatter.getDayCodeFromDateTime(mTaskDateTime))
+            val defaultStartTime = Formatter.getDateTimeFromTS(defaultStartTS)
+
+            mTaskDateTime = mTaskDateTime.withTime(
+                defaultStartTime.hourOfDay,
+                defaultStartTime.minuteOfHour,
+                0,
+                0
+            )
+
+            mConvertedFromOriginalAllDay = true
+            updateTimeText()
+        }
+
         mIsAllDayTask = isChecked
         binding.taskTime.beGoneIf(isChecked)
     }