diff --git a/app/utils/periodic_inform.ts b/app/utils/periodic_inform.ts
new file mode 100644
index 0000000..30126d3
--- /dev/null
+++ b/app/utils/periodic_inform.ts
@@ -0,0 +1,27 @@
+/**
+ * Computes the estimated next periodic inform date.
+ *
+ * @param lastPeriodicInformAt - Date of the last periodic inform
+ * @param periodicInformInterval - Interval in seconds (from device parameter)
+ * @returns The estimated next connection date, or null if cannot be computed
+ */
+export function computeNextPeriodicInform(
+  lastPeriodicInformAt: Date | string | null | undefined,
+  periodicInformInterval: number | string | null | undefined
+): Date | null {
+  if (!lastPeriodicInformAt) return null
+
+  const intervalSeconds = typeof periodicInformInterval === 'string'
+    ? parseInt(periodicInformInterval, 10)
+    : periodicInformInterval
+
+  if (!intervalSeconds || intervalSeconds <= 0) return null
+
+  const lastDate = lastPeriodicInformAt instanceof Date
+    ? lastPeriodicInformAt
+    : new Date(lastPeriodicInformAt)
+
+  if (isNaN(lastDate.getTime())) return null
+
+  return new Date(lastDate.getTime() + intervalSeconds * 1000)
+}
diff --git a/inertia/pages/DeviceDetail.vue b/inertia/pages/DeviceDetail.vue
index b1206c4..e554cba 100644
--- a/inertia/pages/DeviceDetail.vue
+++ b/inertia/pages/DeviceDetail.vue
@@ -310,11 +310,11 @@ async function resolveAllProviders() {
 
               </div>
 
-              <!-- Zones 4+5 : dates côte à côte -->
+              <!-- Zones 4+5+5b : dates sur une ligne -->
               <div class="row g-2">
 
                 <!-- Zone 4 : Créé le -->
-                <div class="col-sm-6">
+                <div class="col-sm-4">
                   <div class="rounded-3 px-3 py-3 d-flex align-items-center gap-3" style="background:#f6f7f9">
                     <div class="rounded-circle d-flex align-items-center justify-content-center flex-shrink-0"
                          style="width:34px;height:34px;background:#e4e6ea">
@@ -330,7 +330,7 @@ async function resolveAllProviders() {
                 </div>
 
                 <!-- Zone 5 : Dernière connexion -->
-                <div class="col-sm-6">
+                <div class="col-sm-4">
                   <div class="rounded-3 px-3 py-3 d-flex align-items-center gap-3" style="background:#f6f7f9">
                     <div class="rounded-circle d-flex align-items-center justify-content-center flex-shrink-0"
                          style="width:34px;height:34px;background:#e4e6ea">
@@ -346,7 +346,7 @@ async function resolveAllProviders() {
                 </div>
 
                 <!-- Zone 5b : Prochaine connexion estimée -->
-                <div v-if="estimatedNextConnection" class="col-sm-6 col-lg-4">
+                <div class="col-sm-4">
                   <div class="rounded-3 px-3 py-3 d-flex align-items-center gap-3" style="background:#f6f7f9">
                     <div class="rounded-circle d-flex align-items-center justify-content-center flex-shrink-0"
                          style="width:34px;height:34px;background:#e4e6ea">
@@ -357,7 +357,7 @@ async function resolveAllProviders() {
                     </div>
                     <div>
                       <div class="text-muted fw-bold text-uppercase mb-1" style="font-size:0.62rem;letter-spacing:.06em">Prochaine connexion</div>
-                      <div class="font-monospace small fw-medium">{{ estimatedNextConnection }}</div>
+                      <div class="font-monospace small fw-medium">{{ estimatedNextConnection || '—' }}</div>
                     </div>
                   </div>
                 </div>
diff --git a/tests/unit/utils/periodic_inform.spec.ts b/tests/unit/utils/periodic_inform.spec.ts
new file mode 100644
index 0000000..ff2e978
--- /dev/null
+++ b/tests/unit/utils/periodic_inform.spec.ts
@@ -0,0 +1,55 @@
+import { describe, it, expect } from 'vitest'
+import { computeNextPeriodicInform } from '../../../app/utils/periodic_inform.js'
+
+describe('computeNextPeriodicInform', () => {
+  it('computes next date from last periodic + interval', () => {
+    const last = new Date('2026-06-02T10:00:00Z')
+    const result = computeNextPeriodicInform(last, 30)
+    expect(result).toEqual(new Date('2026-06-02T10:00:30Z'))
+  })
+
+  it('works with string interval', () => {
+    const last = new Date('2026-06-02T12:00:00Z')
+    const result = computeNextPeriodicInform(last, '60')
+    expect(result).toEqual(new Date('2026-06-02T12:01:00Z'))
+  })
+
+  it('works with ISO string date', () => {
+    const result = computeNextPeriodicInform('2026-06-02T08:00:00Z', 300)
+    expect(result).toEqual(new Date('2026-06-02T08:05:00Z'))
+  })
+
+  it('returns null if lastPeriodicInformAt is null', () => {
+    expect(computeNextPeriodicInform(null, 30)).toBeNull()
+  })
+
+  it('returns null if lastPeriodicInformAt is undefined', () => {
+    expect(computeNextPeriodicInform(undefined, 30)).toBeNull()
+  })
+
+  it('returns null if interval is 0', () => {
+    expect(computeNextPeriodicInform(new Date(), 0)).toBeNull()
+  })
+
+  it('returns null if interval is negative', () => {
+    expect(computeNextPeriodicInform(new Date(), -10)).toBeNull()
+  })
+
+  it('returns null if interval is null', () => {
+    expect(computeNextPeriodicInform(new Date(), null)).toBeNull()
+  })
+
+  it('returns null if interval is empty string', () => {
+    expect(computeNextPeriodicInform(new Date(), '')).toBeNull()
+  })
+
+  it('returns null if date string is invalid', () => {
+    expect(computeNextPeriodicInform('not-a-date', 30)).toBeNull()
+  })
+
+  it('handles large intervals (1 hour)', () => {
+    const last = new Date('2026-06-02T10:00:00Z')
+    const result = computeNextPeriodicInform(last, 3600)
+    expect(result).toEqual(new Date('2026-06-02T11:00:00Z'))
+  })
+})