diff --git a/app/src/main/java/org/nsh07/pomodoro/utils/Utils.kt b/app/src/main/java/org/nsh07/pomodoro/utils/Utils.kt
index 47378e2..2696c04 100644
--- a/app/src/main/java/org/nsh07/pomodoro/utils/Utils.kt
+++ b/app/src/main/java/org/nsh07/pomodoro/utils/Utils.kt
@@ -10,24 +10,30 @@ package org.nsh07.pomodoro.utils
 import java.util.Locale
 import java.util.concurrent.TimeUnit
 
-fun millisecondsToStr(t: Long): String =
-    String.format(
+fun millisecondsToStr(t: Long): String {
+    require(t >= 0L)
+    return String.format(
         Locale.getDefault(),
         "%02d:%02d",
         TimeUnit.MILLISECONDS.toMinutes(t),
         TimeUnit.MILLISECONDS.toSeconds(t) % TimeUnit.MINUTES.toSeconds(1)
     )
+}
 
-fun millisecondsToHours(t: Long): String =
-    String.format(
+fun millisecondsToHours(t: Long): String {
+    require(t >= 0L)
+    return String.format(
         Locale.getDefault(),
         "%dh",
         TimeUnit.MILLISECONDS.toHours(t)
     )
+}
 
-fun millisecondsToHoursMinutes(t: Long): String =
-    String.format(
+fun millisecondsToHoursMinutes(t: Long): String {
+    require(t >= 0L)
+    return String.format(
         Locale.getDefault(),
         "%dh %dm", TimeUnit.MILLISECONDS.toHours(t),
         TimeUnit.MILLISECONDS.toMinutes(t) % TimeUnit.HOURS.toMinutes(1)
-    )
\ No newline at end of file
+    )
+}
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diff --git a/app/src/test/java/org/nsh07/pomodoro/ExampleUnitTest.kt b/app/src/test/java/org/nsh07/pomodoro/ExampleUnitTest.kt
deleted file mode 100644
index fc01855..0000000
--- a/app/src/test/java/org/nsh07/pomodoro/ExampleUnitTest.kt
+++ /dev/null
@@ -1,17 +0,0 @@
-package org.nsh07.pomodoro
-
-import org.junit.Test
-
-import org.junit.Assert.*
-
-/**
- * Example local unit test, which will execute on the development machine (host).
- *
- * See [testing documentation](http://d.android.com/tools/testing).
- */
-class ExampleUnitTest {
-    @Test
-    fun addition_isCorrect() {
-        assertEquals(4, 2 + 2)
-    }
-}
\ No newline at end of file
diff --git a/app/src/test/java/org/nsh07/pomodoro/utils/UtilsKtTest.kt b/app/src/test/java/org/nsh07/pomodoro/utils/UtilsKtTest.kt
new file mode 100644
index 0000000..21391b1
--- /dev/null
+++ b/app/src/test/java/org/nsh07/pomodoro/utils/UtilsKtTest.kt
@@ -0,0 +1,165 @@
+/*
+ * Copyright (c) 2025 Nishant Mishra
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package org.nsh07.pomodoro.utils
+
+import junit.framework.TestCase.assertEquals
+import junit.framework.TestCase.assertTrue
+import org.junit.Assert.assertThrows
+import org.junit.Test
+
+class UtilsKtTest {
+
+    @Test
+    fun `millisecondsToStr zero milliseconds`() {
+        assertEquals("00:00", millisecondsToStr(0))
+    }
+
+    @Test
+    fun `millisecondsToStr positive milliseconds less than a second`() {
+        assertEquals("00:00", millisecondsToStr(999))
+    }
+
+    @Test
+    fun `millisecondsToStr exactly one second`() {
+        assertEquals("00:01", millisecondsToStr(1000))
+    }
+
+    @Test
+    fun `millisecondsToStr positive milliseconds less than a minute`() {
+        assertEquals("00:59", millisecondsToStr(59999))
+    }
+
+    @Test
+    fun `millisecondsToStr exactly one minute`() {
+        assertEquals("01:00", millisecondsToStr(60000))
+    }
+
+    @Test
+    fun `millisecondsToStr positive milliseconds more than a minute`() {
+        assertEquals("01:01", millisecondsToStr(61999))
+    }
+
+    @Test
+    fun `millisecondsToStr large milliseconds value`() {
+        assertEquals("99:59", millisecondsToStr(5999999))
+    }
+
+    @Test
+    fun `millisecondsToStr negative milliseconds`() {
+        val exception = assertThrows(IllegalArgumentException::class.java) {
+            millisecondsToStr(-1)
+        }
+        assertTrue(
+            "millisecondsToStr should throw IllegalArgumentException for negative values",
+            exception is IllegalArgumentException
+        )
+    }
+
+    @Test
+    fun `millisecondsToStr Long MAX VALUE`() {
+        assertEquals("153722867280912:55", millisecondsToStr(Long.MAX_VALUE))
+    }
+
+    @Test
+    fun `millisecondsToHours zero milliseconds`() {
+        assertEquals("0h", millisecondsToHours(0))
+    }
+
+    @Test
+    fun `millisecondsToHours less than an hour`() {
+        assertEquals("0h", millisecondsToHours(3599999))
+    }
+
+    @Test
+    fun `millisecondsToHours exactly one hour`() {
+        assertEquals("1h", millisecondsToHours(3600000))
+    }
+
+    @Test
+    fun `millisecondsToHours multiple hours`() {
+        assertEquals("2h", millisecondsToHours(7200000))
+    }
+
+    @Test
+    fun `millisecondsToHours large number of hours`() {
+        assertEquals("25h", millisecondsToHours(25 * 3600000))
+    }
+
+    @Test
+    fun `millisecondsToHours negative milliseconds`() {
+        val exception = assertThrows(IllegalArgumentException::class.java) {
+            millisecondsToHours(-1)
+        }
+        assertTrue(
+            "millisecondsToHours should throw IllegalArgumentException for negative values",
+            exception is IllegalArgumentException
+        )
+    }
+
+    @Test
+    fun `millisecondsToHours Long MAX VALUE`() {
+        assertEquals("2562047788015h", millisecondsToHours(Long.MAX_VALUE))
+    }
+
+    @Test
+    fun `millisecondsToHoursMinutes zero milliseconds`() {
+        assertEquals("0h 0m", millisecondsToHoursMinutes(0))
+    }
+
+    @Test
+    fun `millisecondsToHoursMinutes less than a minute`() {
+        assertEquals("0h 0m", millisecondsToHoursMinutes(59999))
+    }
+
+    @Test
+    fun `millisecondsToHoursMinutes exactly one minute`() {
+        assertEquals("0h 1m", millisecondsToHoursMinutes(60000))
+    }
+
+    @Test
+    fun `millisecondsToHoursMinutes less than an hour but more than a minute`() {
+        assertEquals("0h 59m", millisecondsToHoursMinutes(3599999))
+    }
+
+    @Test
+    fun `millisecondsToHoursMinutes exactly one hour`() {
+        assertEquals("1h 0m", millisecondsToHoursMinutes(3600000))
+    }
+
+    @Test
+    fun `millisecondsToHoursMinutes hours and minutes`() {
+        assertEquals("1h 1m", millisecondsToHoursMinutes(3660000))
+    }
+
+    @Test
+    fun `millisecondsToHoursMinutes multiple hours and minutes`() {
+        assertEquals("2h 3m", millisecondsToHoursMinutes(7380000))
+    }
+
+    @Test
+    fun `millisecondsToHoursMinutes just less than 2 hours`() {
+        assertEquals("1h 59m", millisecondsToHoursMinutes(3600000 + 3599999))
+    }
+
+    @Test
+    fun `millisecondsToHoursMinutes negative milliseconds`() {
+        val exception = assertThrows(IllegalArgumentException::class.java) {
+            millisecondsToHoursMinutes(-1)
+        }
+        assertTrue(
+            "millisecondsToHoursMinutes should throw IllegalArgumentException for negative values",
+            exception is IllegalArgumentException
+        )
+    }
+
+    @Test
+    fun `millisecondsToHoursMinutes Long MAX VALUE`() {
+        assertEquals("2562047788015h 12m", millisecondsToHoursMinutes(Long.MAX_VALUE))
+    }
+
+}
\ No newline at end of file