diff --git a/src/Service/AbsenceDayCalculator.php b/src/Service/AbsenceDayCalculator.php
index 2a4d667..d30a598 100644
--- a/src/Service/AbsenceDayCalculator.php
+++ b/src/Service/AbsenceDayCalculator.php
@@ -54,11 +54,19 @@ final readonly class AbsenceDayCalculator
         // A half-day only subtracts 0.5 when its boundary day is actually
         // counted (otherwise a half-day posted on a weekend/holiday would
         // wrongly under-count the absence).
-        if (null !== $startHalfDay && $this->isCountedDay($start, $workingDaysOnly)) {
-            $days -= 0.5;
-        }
-        if (null !== $endHalfDay && $this->isCountedDay($end, $workingDaysOnly)) {
-            $days -= 0.5;
+        if ($start->getTimestamp() === $end->getTimestamp()) {
+            // Single-day request: both boundaries collapse onto the same day,
+            // so a half-day must subtract 0.5 once, never twice.
+            if ((null !== $startHalfDay || null !== $endHalfDay) && $this->isCountedDay($start, $workingDaysOnly)) {
+                $days -= 0.5;
+            }
+        } else {
+            if (null !== $startHalfDay && $this->isCountedDay($start, $workingDaysOnly)) {
+                $days -= 0.5;
+            }
+            if (null !== $endHalfDay && $this->isCountedDay($end, $workingDaysOnly)) {
+                $days -= 0.5;
+            }
         }
 
         return max(0.0, $days);
diff --git a/tests/Unit/Service/AbsenceDayCalculatorTest.php b/tests/Unit/Service/AbsenceDayCalculatorTest.php
index 0eecdcd..33b96a3 100644
--- a/tests/Unit/Service/AbsenceDayCalculatorTest.php
+++ b/tests/Unit/Service/AbsenceDayCalculatorTest.php
@@ -68,6 +68,15 @@ class AbsenceDayCalculatorTest extends TestCase
         ));
     }
 
+    public function testSingleFullDayCountsOne(): void
+    {
+        // Most common request: a single full working day = 1.0 (no half-day).
+        self::assertSame(1.0, $this->calculator->countWorkingDays(
+            new DateTimeImmutable('2026-06-01'),
+            new DateTimeImmutable('2026-06-01'),
+        ));
+    }
+
     public function testSingleHalfDay(): void
     {
         self::assertSame(0.5, $this->calculator->countWorkingDays(
@@ -77,6 +86,73 @@ class AbsenceDayCalculatorTest extends TestCase
         ));
     }
 
+    public function testHalfDayEndOnlySubtractsHalf(): void
+    {
+        // Mirror of testHalfDayStartSubtractsHalf: end half-day only, on a
+        // counted multi-day range => 5 - 0.5 = 4.5.
+        self::assertSame(4.5, $this->calculator->countWorkingDays(
+            new DateTimeImmutable('2026-06-01'),
+            new DateTimeImmutable('2026-06-05'),
+            null,
+            HalfDay::Morning,
+        ));
+    }
+
+    public function testEndBeforeStartCountsZero(): void
+    {
+        // Inverted range guard (end < start) => 0, never negative.
+        self::assertSame(0.0, $this->calculator->countWorkingDays(
+            new DateTimeImmutable('2026-06-05'),
+            new DateTimeImmutable('2026-06-01'),
+        ));
+    }
+
+    public function testSingleDayOnWeekendCountsZero(): void
+    {
+        // Saturday, jours ouvrés: not counted, even with a half-day => 0.
+        self::assertSame(0.0, $this->calculator->countWorkingDays(
+            new DateTimeImmutable('2026-06-06'),
+            new DateTimeImmutable('2026-06-06'),
+            HalfDay::Morning,
+            HalfDay::Morning,
+        ));
+    }
+
+    public function testSingleDayOnHolidayCountsZero(): void
+    {
+        // Fri 2026-05-08 (Victoire 1945): public holiday => 0.
+        self::assertSame(0.0, $this->calculator->countWorkingDays(
+            new DateTimeImmutable('2026-05-08'),
+            new DateTimeImmutable('2026-05-08'),
+        ));
+    }
+
+    public function testHalfDayOnSaturdayCountedInOpenDays(): void
+    {
+        // Fri + Sat in "ouvrables" mode = 2; the end half-day sits on Saturday,
+        // which IS counted here => 2 - 0.5 = 1.5.
+        self::assertSame(1.5, $this->calculator->countWorkingDays(
+            new DateTimeImmutable('2026-06-05'),
+            new DateTimeImmutable('2026-06-06'),
+            null,
+            HalfDay::Morning,
+            false,
+        ));
+    }
+
+    public function testSingleDayWithBothHalfDaysIsStillHalf(): void
+    {
+        // Real-world single-day request: the form mirrors the start half-day
+        // onto the end (same day), so both boundaries carry a half-day. It must
+        // still count as 0.5, not 0 (the two boundaries collapse onto one day).
+        self::assertSame(0.5, $this->calculator->countWorkingDays(
+            new DateTimeImmutable('2026-06-01'),
+            new DateTimeImmutable('2026-06-01'),
+            HalfDay::Morning,
+            HalfDay::Morning,
+        ));
+    }
+
     public function testHalfDayOnNonCountedStartIsIgnored(): void
     {
         // Sat 2026-06-06 → Mon 2026-06-08, jours ouvrés : Sat & Sun skipped, Mon = 1.