chore(test): add idependent tests for time util component

src/test/kotlin/io/github/devcavin/gatelog/common/time/TimeUtilTest.kt

@@ -0,0 +1,41 @@
+package io.github.devcavin.gatelog.common.time
+
+import org.junit.jupiter.api.Test
+import java.time.Clock
+import java.time.OffsetDateTime
+import kotlin.test.assertFalse
+import kotlin.test.assertTrue
+
+class TimeUtilTest {
+    val clock: Clock = Clock.systemDefaultZone()
+    val timeUtil = TimeUtil(clock)
+
+    @Test
+    fun `checkin time yesterday is overnight`() {
+        val checkinTime = OffsetDateTime.parse("2026-09-26T20:00:00Z")
+        val now = OffsetDateTime.parse("2026-09-27T00:00:00Z")
+
+        val result = timeUtil.isOvernight(checkinTime, now)
+
+        assertTrue(result)
+    }
+
+    @Test
+    fun `checkin time today is not overnight`() {
+        val checkinTime = OffsetDateTime.parse("2026-09-26T20:00:00Z")
+        val now = OffsetDateTime.parse("2026-09-26T23:59:59Z")
+
+        val result = timeUtil.isOvernight(checkinTime, now)
+
+        assertFalse(result)
+    }
+
+    @Test
+    fun `checkin time previous day is overnight`() {
+        val checkinTime = OffsetDateTime.parse("2026-09-24T23:59:59Z")
+        val now = OffsetDateTime.parse("2026-09-25T00:00:00Z")
+
+        val result = timeUtil.isOvernight(checkinTime, now)
+        assertTrue(result)
+    }
+}