diff --git a/app/src/main/kotlin/br/com/colman/petals/hittimer/HitTimer.kt b/app/src/main/kotlin/br/com/colman/petals/hittimer/HitTimer.kt
index b99eaed6..63179c0a 100644
--- a/app/src/main/kotlin/br/com/colman/petals/hittimer/HitTimer.kt
+++ b/app/src/main/kotlin/br/com/colman/petals/hittimer/HitTimer.kt
@@ -33,9 +33,10 @@ class HitTimer(val durationMillis: Long = 10_000L) : Parcelable {
     startDate = null
   }
 
+  @Suppress("UnsafeCallOnNullableType")
   private fun calculateMillisLeft(): Long {
     if (startDate == null) return durationMillis
-    val elapsed = startDate?.until(now(), MILLIS) ?: 0
+    val elapsed = startDate!!.until(now(), MILLIS)
     return (durationMillis - elapsed).coerceAtLeast(0)
   }
 
diff --git a/app/src/test/kotlin/br/com/colman/petals/hittimer/HitTimerTest.kt b/app/src/test/kotlin/br/com/colman/petals/hittimer/HitTimerTest.kt
index 2f41a050..0a5c3470 100644
--- a/app/src/test/kotlin/br/com/colman/petals/hittimer/HitTimerTest.kt
+++ b/app/src/test/kotlin/br/com/colman/petals/hittimer/HitTimerTest.kt
@@ -5,8 +5,12 @@ import io.kotest.datatest.withData
 import io.kotest.inspectors.forAll
 import io.kotest.matchers.collections.shouldBeMonotonicallyDecreasing
 import io.kotest.matchers.ints.shouldBeGreaterThanOrEqual
+import io.kotest.matchers.longs.shouldBeLessThanOrEqual
 import io.kotest.matchers.shouldBe
+import kotlinx.coroutines.delay
+import kotlinx.coroutines.flow.first
 import kotlinx.coroutines.flow.take
+import kotlinx.coroutines.flow.takeWhile
 import kotlinx.coroutines.flow.toList
 
 class HitTimerTest : FunSpec({
@@ -35,17 +39,72 @@ class HitTimerTest : FunSpec({
     allResults.size shouldBeGreaterThanOrEqual (duration / 10).toInt()
   }
 
-  context("Duration with milliseconds disabled should not show 0 while there are still milliseconds") {
+  context("Error Cases") {
+    test("Should return max duration if timer wasn't started") {
+      target.millisLeft.first() shouldBe duration
+    }
+  }
+
+  context("Reset functionality") {
+    test("Should reset to initial duration when reset is called after starting") {
+      target.start()
+      target.reset()
+      target.millisLeft.first() shouldBe duration
+    }
+
+    test("After timer completes, reset should restore duration") {
+      target.start()
+      target.millisLeft.takeWhile { it > 0 }.toList()
+      target.reset()
+      target.millisLeft.first() shouldBe duration
+    }
+  }
+
+  context("Start multiple times") {
+    test("Calling start multiple times resets the timer") {
+      target.start()
+      delay(20L)
+      val firstValue = target.millisLeft.first()
+      target.start()
+      val secondValue = target.millisLeft.first()
+
+      secondValue shouldBe duration
+      firstValue shouldBeLessThanOrEqual (duration - 20)
+    }
+  }
+
+  context("Duration formatting") {
     withData(
-      nameFn = { (millis, string) -> "$millis milliseconds should be converted to $string" },
-      0L to "0.0",
-      100L to "0.1",
-      249L to "0.2",
-      250L to "0.3",
-      666L to "0.7",
-      1200L to "1"
+      nameFn = { (millis, expected) -> "$millis ms -> $expected" },
+      0L to "00:000",
+      1000L to "01:000",
+      1234L to "01:234",
+      9999L to "09:999",
+      10_000L to "10:000"
+    ) { (millis, expected) ->
+      HitTimer.duration(millis) shouldBe expected
+    }
+  }
+
+  context("Duration with milliseconds disabled edge cases") {
+    withData(
+      nameFn = { (millis, string) -> "$millis ms -> $string" },
+      999L to "1.0",
+      1000L to "1.0",
+      1001L to "1",
+      500L to "0.5",
+      499L to "0.5",
+      1L to "0.0",
+      0L to "0.0"
     ) { (millis, string) ->
       HitTimer.durationMillisecondsDisabled(millis) shouldBe string
     }
   }
+
+  test("Timer stays at zero after duration has passed") {
+    target.start()
+    delay(duration)
+    target.millisLeft.first() shouldBe 0
+    target.millisLeft.take(3).toList().forAll { it shouldBe 0 }
+  }
 })