Add TimeSource toClock converter

core/common/src/Clock.kt

@@ -35,3 +35,17 @@ private class InstantTimeMark(private val instant: Instant, private val clock: C
 
     override fun minus(duration: Duration): TimeMark = InstantTimeMark(instant - duration, clock)
 }
+
+/**
+ * Returns the [Clock] by storing an initial [TimeMark] using [TimeSource.markNow] and [returns][Clock.now] the elapsed
+ * time using [TimeMark.elapsedNow] plus the provided [offset].
+ *
+ * This clock stores the initial [TimeMark], so repeatedly creating [Clock]s from the same [TimeSource] results
+ * into different [Instant]s iff the time of the [TimeSource] was increased. To sync different [Clock]s, use the [offset]
+ * parameter.
+ */
+@ExperimentalTime
+public fun TimeSource.toClock(offset: Instant = Instant.fromEpochSeconds(0)): Clock = object : Clock {
+    private val startMark: TimeMark = markNow()
+    override fun now() = offset + startMark.elapsedNow()
+}

core/common/test/ClockTest.kt

@@ -0,0 +1,51 @@
+/*
+ * Copyright 2019-2021 JetBrains s.r.o.
+ * Use of this source code is governed by the Apache 2.0 License that can be found in the LICENSE.txt file.
+ */
+
+package kotlinx.datetime.test
+
+import kotlinx.datetime.*
+import kotlin.test.*
+import kotlin.time.*
+import kotlin.time.Duration.Companion.seconds
+
+@ExperimentalTime
+class ClockTest {
+    @Test
+    fun timeSourceToClock() {
+        val timeSource = TestTimeSource()
+        val clock = timeSource.toClock()
+
+        assertEquals(Instant.fromEpochSeconds(0), clock.now())
+        assertEquals(Instant.fromEpochSeconds(0), clock.now())
+
+        timeSource += 1.seconds
+        assertEquals(Instant.fromEpochSeconds(1), clock.now())
+        assertEquals(Instant.fromEpochSeconds(1), clock.now())
+    }
+
+    @Test
+    fun syncMultipleClocksFromTimeSource() {
+        val timeSource = TestTimeSource()
+        val clock1 = timeSource.toClock()
+
+        assertEquals(0, clock1.now().epochSeconds)
+
+        timeSource += 1.seconds
+        assertEquals(1, clock1.now().epochSeconds)
+
+        val clock2 = timeSource.toClock(offset = Instant.fromEpochSeconds(1))
+        assertEquals(clock1.now(), clock2.now())
+
+        timeSource += 1.seconds
+        assertEquals(2, clock1.now().epochSeconds)
+        assertEquals(clock1.now(), clock2.now())
+
+        val clock3 = timeSource.toClock(offset = clock2.now())
+        timeSource += 1.seconds
+        assertEquals(3, clock3.now().epochSeconds)
+        assertEquals(clock1.now(), clock2.now())
+        assertEquals(clock1.now(), clock3.now())
+    }
+}