Use MockK to fix the system clock

ClockFixingTest.kt

import io.mockk.every
import io.mockk.mockkStatic
import org.junit.jupiter.api.Assertions.assertEquals
import org.junit.jupiter.api.BeforeEach
import org.junit.jupiter.api.Test
import java.time.Clock
import java.time.Instant
import java.time.ZoneId

internal class ClockFixingTest {

    private val now = 1550160535168L
    private val fixedClock = Clock.fixed(Instant.ofEpochMilli(now), ZoneId.systemDefault())

    @BeforeEach
    fun `fix the clock =)`() {
        mockkStatic(Clock::class)
        // Default system clock
        every { Clock.systemUTC() } returns fixedClock
    }

    @Test
    fun `can fix clock`() {
        assertEquals(now, Instant.now().toEpochMilli())
    }
}