Fix off-by-one error in fromNowUntilNext

modules/core/src/main/scala/eu/timepit/fs2cron/ZonedDateTimeScheduler.scala

@@ -17,7 +17,7 @@
 package eu.timepit.fs2cron
 
 import cats.effect.{Sync, Temporal}
-import cats.syntax.all._
+import cats.syntax.all.*
 
 import java.time.temporal.ChronoUnit
 import java.time.{Instant, ZoneId, ZoneOffset, ZonedDateTime}
@@ -29,14 +29,20 @@ abstract class ZonedDateTimeScheduler[F[_], Schedule](zoneId: F[ZoneId])(implici
 ) extends Scheduler[F, Schedule] {
   def next(from: ZonedDateTime, schedule: Schedule): F[ZonedDateTime]
 
-  override def fromNowUntilNext(schedule: Schedule): F[FiniteDuration] =
-    now.flatMap { from =>
-      next(from, schedule).map { to =>
-        val durationInMillis = from.until(to, ChronoUnit.MILLIS)
-        FiniteDuration(durationInMillis, TimeUnit.MILLISECONDS)
-      }
+  def durationUntilNext(from: ZonedDateTime, schedule: Schedule): F[FiniteDuration] =
+    next(from, schedule).map { to =>
+      val durationInMillis = from
+        // Since `until` only returns complete units between `from` and `to`,
+        // we truncate `from` to milliseconds, so that `durationInMillis` + `from`
+        // is never before `to`. See https://github.com/fthomas/fs2-cron/issues/598.
+        .truncatedTo(ChronoUnit.MILLIS)
+        .until(to, ChronoUnit.MILLIS)
+      FiniteDuration(durationInMillis, TimeUnit.MILLISECONDS)
     }
 
+  override def fromNowUntilNext(schedule: Schedule): F[FiniteDuration] =
+    now.flatMap(from => durationUntilNext(from, schedule))
+
   private val now: F[ZonedDateTime] =
     (temporal.realTime, zoneId).mapN((d, z) => Instant.EPOCH.plusNanos(d.toNanos).atZone(z))
 }

modules/core/src/test/scala/eu/timepit/fs2cron/ZonedDateTimeSchedulerSuite.scala

@@ -20,7 +20,9 @@ import cats.effect.IO
 import fs2.Stream
 import munit.CatsEffectSuite
 
-import java.time.{Instant, ZoneId, ZoneOffset}
+import java.time.{Instant, ZoneId, ZoneOffset, ZonedDateTime}
+import java.util.concurrent.TimeUnit
+import scala.concurrent.duration.FiniteDuration
 
 trait ZonedDateTimeSchedulerSuite[Schedule] extends CatsEffectSuite {
   def schedulerCompanion: ZonedDateTimeScheduler.Companion[Schedule]
@@ -76,4 +78,20 @@ trait ZonedDateTimeSchedulerSuite[Schedule] extends CatsEffectSuite {
     val s2 = s1.map(instantSeconds).take(2).forall(!isEven(_))
     assertIO(s2.compile.last, Some(true))
   }
+
+  test("durationUntilNext: from + duration >= next") {
+    val scheduler = schedulerUtc.asInstanceOf[ZonedDateTimeScheduler[IO, Schedule]]
+
+    val from = ZonedDateTime.of(1970, 1, 1, 0, 0, 0, 100000, ZoneOffset.UTC)
+    val duration = scheduler.durationUntilNext(from, everySecond)
+
+    val next = scheduler.next(from, everySecond)
+    val fromPlusDuration = duration.map(d => from.plusNanos(d.toNanos))
+    val fromPlusDurationGtEqNext = fromPlusDuration.flatMap(fpd => next.map(n => !fpd.isBefore(n)))
+
+    for {
+      _ <- assertIO(duration, FiniteDuration(1000L, TimeUnit.MILLISECONDS))
+      _ <- assertIOBoolean(fromPlusDurationGtEqNext)
+    } yield ()
+  }
 }