Add convenience method for Clock testing

dachlatten-datetime/build.gradle.kts

@@ -7,4 +7,6 @@ plugins {
 dependencies {
     compileOnly(libs.kotlinx.datetime)
     compileOnly(libs.android.support.annotations)
+
+    testImplementation(libs.kotlinx.datetime)
 }

dachlatten-datetime/src/main/kotlin/de/sipgate/dachlatten/datetime/FixedClock.kt

@@ -0,0 +1,8 @@
+package de.sipgate.dachlatten.datetime
+
+import kotlinx.datetime.Clock
+import kotlinx.datetime.Instant
+
+fun Clock.Companion.Fixed(instant: Instant) = object : Clock {
+    override fun now(): Instant = instant
+}

dachlatten-datetime/src/test/kotlin/de/sipgate/dachlatten/datetime/FixedClockTest.kt

@@ -0,0 +1,30 @@
+package de.sipgate.dachlatten.datetime
+
+import kotlinx.datetime.Clock
+import kotlinx.datetime.Instant
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Test
+
+class FixedClockTest {
+    @Test
+    fun fixedClockReturnsFixedInstance() {
+        val instant = Instant.fromEpochSeconds(1713453869)
+        val fixedClock = Clock.Fixed(instant)
+
+        val instantFromClock = fixedClock.now()
+
+        assertEquals(instant, instantFromClock)
+    }
+
+    @Test
+    fun fixedClockReturnsFixedInstanceWhenCalledMultipleTimes() {
+        val instant = Instant.fromEpochSeconds(1713453869)
+        val fixedClock = Clock.Fixed(instant)
+
+        val firstInstantFromClock = fixedClock.now()
+        val secondInstantFromClock = fixedClock.now()
+
+        assertEquals(instant, firstInstantFromClock)
+        assertEquals(firstInstantFromClock, secondInstantFromClock)
+    }
+}