Challenge 1: A Basic expect/actual Pair — Solution

// ── commonMain/kotlin/Timestamp.kt ──
expect fun currentTimestamp(): Long

fun describeTimestamp(): String = "Current timestamp: ${currentTimestamp()}"

// ── androidMain/kotlin/Timestamp.kt ──
actual fun currentTimestamp(): Long = System.currentTimeMillis()

// ── jvmMain/kotlin/Timestamp.kt ──
actual fun currentTimestamp(): Long = System.currentTimeMillis()

fun main() {
    println(describeTimestamp())
}

/*
Output (value will differ each run):
Current timestamp: 1751476800000

Notes:
- currentTimestamp() is declared "expect" in commonMain with no body —
  it's a promise that every target platform will supply an implementation,
  not an implementation itself.
- Both actual implementations happen to use the same
  System.currentTimeMillis() call here since Android and plain JVM share
  the same java.lang.System API — but they're still two separate actual
  declarations, one per source set, and could diverge (e.g. iOS would
  need a completely different API, like NSDate, instead).
- describeTimestamp() lives entirely in commonMain and calls
  currentTimestamp() without knowing or caring which actual it will be
  compiled against — that resolution happens per-platform at compile time.
*/