diff --git a/packages/code-du-travail-modeles/src/modeles/common/utils/__tests__/accumulate-absence-by-year.spec.ts b/packages/code-du-travail-modeles/src/modeles/common/utils/__tests__/accumulate-absence-by-year.spec.ts
index b6c48f8a77..91070e221e 100644
--- a/packages/code-du-travail-modeles/src/modeles/common/utils/__tests__/accumulate-absence-by-year.spec.ts
+++ b/packages/code-du-travail-modeles/src/modeles/common/utils/__tests__/accumulate-absence-by-year.spec.ts
@@ -15,7 +15,7 @@ describe("Accumulation d'absence par année", () => {
     ${[{ durationInMonth: 1, motif: { value: 1 }, startedAt: "02/06/2020" }]}                                                                       | ${[{ begin: date2020, end: exitIn2020 }]}                                               | ${[{ totalAbsenceInMonth: 1, year: 2020 }]}
     ${[{ durationInMonth: 1, motif: { value: 1 }, startedAt: "10/05/2020" }, { durationInMonth: 2, motif: { value: 1 }, startedAt: "10/07/2020" }]} | ${[{ begin: date2020, end: exitIn2020 }]}                                               | ${[{ totalAbsenceInMonth: 3, year: 2020 }]}
     ${[{ durationInMonth: 3, motif: { value: 1 }, startedAt: "10/07/2020" }, { durationInMonth: 2, motif: { value: 1 }, startedAt: "10/04/2021" }]} | ${[{ begin: date2020, end: date2021LessOneDay }, { begin: date2021, end: exitIn2021 }]} | ${[{ totalAbsenceInMonth: 3, year: 2020 }, { totalAbsenceInMonth: 2, year: 2021 }]}
-    ${[{ durationInMonth: 1, motif: { value: 1 }, startedAt: "20/03/2021" }]}                                                                       | ${[{ begin: date2020, end: date2021LessOneDay }, { begin: date2021, end: exitIn2021 }]} | ${[{ totalAbsenceInMonth: 0.4666666666666667, year: 2020 }, { totalAbsenceInMonth: 0.4666666666666667, year: 2021 }]}
+    ${[{ durationInMonth: 1, motif: { value: 1 }, startedAt: "20/03/2021" }]}                                                                       | ${[{ begin: date2020, end: date2021LessOneDay }, { begin: date2021, end: exitIn2021 }]} | ${[{ totalAbsenceInMonth: 0.5, year: 2020 }, { totalAbsenceInMonth: 0.5, year: 2021 }]}
   `(
     "Avec $absences.length absence(s) et $years.length année(s) on attend $expectedOutput.length découpe(s) par année",
     ({ absences, years, expectedOutput }) => {
diff --git a/packages/code-du-travail-modeles/src/modeles/common/utils/seniority.ts b/packages/code-du-travail-modeles/src/modeles/common/utils/seniority.ts
index 99a9fafa23..3262530278 100644
--- a/packages/code-du-travail-modeles/src/modeles/common/utils/seniority.ts
+++ b/packages/code-du-travail-modeles/src/modeles/common/utils/seniority.ts
@@ -1,6 +1,7 @@
 import type { Interval } from "date-fns";
 import {
   add,
+  addDays,
   addMonths,
   areIntervalsOverlapping,
   differenceInCalendarMonths,
@@ -170,7 +171,11 @@ const getOverlappingMonthsInterval = (r1: Interval, r2: Interval) => {
     const end = min([r1.end, r2.end]);
     const diffInMonths = differenceInCalendarMonths(end, start);
     const startDateWithDifferenceInMonths = addMonths(start, diffInMonths);
-    const diffInDays = differenceInDays(end, startDateWithDifferenceInMonths);
+    const endWithOneDay = addDays(end, 1);
+    const diffInDays = differenceInDays(
+      endWithOneDay,
+      startDateWithDifferenceInMonths
+    );
     return diffInMonths + diffInDays / 30;
   } else return 0;
 };