diff --git a/library/core-utils/build.gradle b/library/core-utils/build.gradle
index b985127ec6..d1e82fcb9a 100644
--- a/library/core-utils/build.gradle
+++ b/library/core-utils/build.gradle
@@ -53,4 +53,12 @@ android {
 
 dependencies {
     implementation libs.jetbrains.coroutinesAndroid
+
+    // TESTS
+    testImplementation libs.tests.junit
+    testImplementation libs.tests.kluent
+    testImplementation libs.mockk.mockk
+    testImplementation(libs.jetbrains.coroutinesTest) {
+        exclude group: "org.jetbrains.kotlinx", module: "kotlinx-coroutines-debug"
+    }
 }
diff --git a/library/core-utils/src/main/java/im/vector/lib/core/utils/timer/CountUpTimer.kt b/library/core-utils/src/main/java/im/vector/lib/core/utils/timer/CountUpTimer.kt
index 8485398ffa..435a8603e8 100644
--- a/library/core-utils/src/main/java/im/vector/lib/core/utils/timer/CountUpTimer.kt
+++ b/library/core-utils/src/main/java/im/vector/lib/core/utils/timer/CountUpTimer.kt
@@ -19,22 +19,21 @@ package im.vector.lib.core.utils.timer
 import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.Job
-import kotlinx.coroutines.cancel
 import kotlinx.coroutines.delay
 import kotlinx.coroutines.launch
 import java.util.concurrent.atomic.AtomicLong
 
 @OptIn(kotlinx.coroutines.ExperimentalCoroutinesApi::class)
 class CountUpTimer(
+        private val coroutineScope: CoroutineScope = CoroutineScope(Dispatchers.Main),
         private val clock: Clock = DefaultClock(),
         private val intervalInMs: Long = 1_000,
         initialTime: Long = 0L,
 ) {
 
-    private val coroutineScope = CoroutineScope(Dispatchers.Main)
     private var counterJob: Job? = null
 
-    private val lastTime: AtomicLong = AtomicLong()
+    private val lastTime: AtomicLong = AtomicLong(clock.epochMillis())
     private val elapsedTime: AtomicLong = AtomicLong(initialTime)
 
     private fun startCounter() {
@@ -70,7 +69,7 @@ class CountUpTimer(
 
     fun stop() {
         tickListener?.onTick(elapsedTime())
-        coroutineScope.cancel()
+        counterJob?.cancel()
         counterJob = null
     }
 
diff --git a/library/core-utils/src/test/java/im/vector/lib/core/utils/test/fakes/FakeClock.kt b/library/core-utils/src/test/java/im/vector/lib/core/utils/test/fakes/FakeClock.kt
new file mode 100644
index 0000000000..4bad4471f1
--- /dev/null
+++ b/library/core-utils/src/test/java/im/vector/lib/core/utils/test/fakes/FakeClock.kt
@@ -0,0 +1,27 @@
+/*
+ * Copyright (c) 2023 New Vector Ltd
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package im.vector.lib.core.utils.test.fakes
+
+import im.vector.lib.core.utils.timer.Clock
+import io.mockk.every
+import io.mockk.mockk
+
+class FakeClock : Clock by mockk() {
+    fun givenEpoch(epoch: Long) {
+        every { epochMillis() } returns epoch
+    }
+}
diff --git a/library/core-utils/src/test/java/im/vector/lib/core/utils/timer/CountUpTimerTest.kt b/library/core-utils/src/test/java/im/vector/lib/core/utils/timer/CountUpTimerTest.kt
new file mode 100644
index 0000000000..df517751ed
--- /dev/null
+++ b/library/core-utils/src/test/java/im/vector/lib/core/utils/timer/CountUpTimerTest.kt
@@ -0,0 +1,96 @@
+/*
+ * Copyright (c) 2023 New Vector Ltd
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package im.vector.lib.core.utils.timer
+
+import im.vector.lib.core.utils.test.fakes.FakeClock
+import io.mockk.every
+import io.mockk.mockk
+import io.mockk.verifySequence
+import kotlinx.coroutines.ExperimentalCoroutinesApi
+import kotlinx.coroutines.test.advanceTimeBy
+import kotlinx.coroutines.test.currentTime
+import kotlinx.coroutines.test.runTest
+import org.junit.Test
+
+private const val AN_INTERVAL = 500L
+private const val AN_INITIAL_TIME = 2_333L
+
+@OptIn(ExperimentalCoroutinesApi::class)
+internal class CountUpTimerTest {
+
+    private val fakeClock = FakeClock()
+
+    @Test
+    fun `when pausing and resuming the timer, the timer ticks the right values at the right moments`() = runTest {
+        every { fakeClock.epochMillis() } answers { currentTime }
+        val tickListener = mockk<CountUpTimer.TickListener>(relaxed = true)
+        val timer = CountUpTimer(
+                coroutineScope = this,
+                clock = fakeClock,
+                intervalInMs = AN_INTERVAL,
+                initialTime = 0,
+        ).also { it.tickListener = tickListener }
+
+        timer.resume()
+        advanceTimeBy(AN_INTERVAL / 2) // no tick
+        timer.pause() // tick
+        advanceTimeBy(AN_INTERVAL * 10) // no tick
+        timer.resume() // no tick
+        advanceTimeBy(AN_INTERVAL * 4) // tick * 4
+        timer.stop() // tick
+
+        verifySequence {
+            tickListener.onTick(AN_INTERVAL / 2)
+            tickListener.onTick(AN_INTERVAL)
+            tickListener.onTick(AN_INTERVAL * 2)
+            tickListener.onTick(AN_INTERVAL * 3)
+            tickListener.onTick(AN_INTERVAL * 4)
+            tickListener.onTick(AN_INTERVAL * 4 + AN_INTERVAL / 2)
+        }
+    }
+
+    @Test
+    fun `given an initial time, the timer ticks the right values at the right moments`() = runTest {
+        every { fakeClock.epochMillis() } answers { currentTime }
+        val tickListener = mockk<CountUpTimer.TickListener>(relaxed = true)
+        val timer = CountUpTimer(
+                coroutineScope = this,
+                clock = fakeClock,
+                intervalInMs = AN_INTERVAL,
+                initialTime = AN_INITIAL_TIME,
+        ).also { it.tickListener = tickListener }
+
+        timer.resume()
+        advanceTimeBy(AN_INTERVAL) // tick
+        timer.pause() // tick
+        advanceTimeBy(AN_INTERVAL * 10) // no tick
+        timer.resume() // no tick
+        advanceTimeBy(AN_INTERVAL * 4) // tick * 4
+        timer.stop() // tick
+
+        val offset = AN_INITIAL_TIME % AN_INTERVAL
+        verifySequence {
+            tickListener.onTick(AN_INITIAL_TIME + AN_INTERVAL - offset)
+            tickListener.onTick(AN_INITIAL_TIME + AN_INTERVAL)
+            tickListener.onTick(AN_INITIAL_TIME + AN_INTERVAL * 2 - offset)
+            tickListener.onTick(AN_INITIAL_TIME + AN_INTERVAL * 3 - offset)
+            tickListener.onTick(AN_INITIAL_TIME + AN_INTERVAL * 4 - offset)
+            tickListener.onTick(AN_INITIAL_TIME + AN_INTERVAL * 5 - offset)
+            tickListener.onTick(AN_INITIAL_TIME + AN_INTERVAL * 5)
+        }
+    }
+}