fix: atTime being incorrectly set for non compatible units

job.go

@@ -252,10 +252,14 @@ func (j *Job) addAtTime(t time.Duration) {
 }
 
 func (j *Job) getStartAtTime() time.Time {
+	j.mu.RLock()
+	defer j.mu.RUnlock()
 	return j.startAtTime
 }
 
 func (j *Job) setStartAtTime(t time.Time) {
+	j.mu.Lock()
+	defer j.mu.Unlock()
 	j.startAtTime = t
 }
 

scheduler.go

@@ -205,15 +205,15 @@ func (s *Scheduler) scheduleNextRun(job *Job) (bool, nextRun) {
 		// Increment startAtTime to the future
 		if !job.startAtTime.IsZero() && job.startAtTime.Before(now) {
 			duration := s.durationToNextRun(job.startAtTime, job).duration
-			job.startAtTime = job.startAtTime.Add(duration)
+			job.setStartAtTime(job.startAtTime.Add(duration))
 			if job.startAtTime.Before(now) {
 				diff := now.Sub(job.startAtTime)
 				duration := s.durationToNextRun(job.startAtTime, job).duration
 				count := diff / duration
 				if diff%duration != 0 {
 					count++
 				}
-				job.startAtTime = job.startAtTime.Add(duration * count)
+				job.setStartAtTime(job.startAtTime.Add(duration * count))
 			}
 		}
 	} else {
@@ -248,12 +248,14 @@ func (s *Scheduler) durationToNextRun(lastRun time.Time, job *Job) nextRun {
 	// job can be scheduled with .StartAt()
 	if job.getFirstAtTime() == 0 && job.getStartAtTime().After(lastRun) {
 		sa := job.getStartAtTime()
-		job.addAtTime(
-			time.Duration(sa.Hour())*time.Hour +
-				time.Duration(sa.Minute())*time.Minute +
-				time.Duration(sa.Second())*time.Second,
-		)
-		return nextRun{duration: job.getStartAtTime().Sub(s.now()), dateTime: job.getStartAtTime()}
+		if job.unit == days || job.unit == weeks || job.unit == months {
+			job.addAtTime(
+				time.Duration(sa.Hour())*time.Hour +
+					time.Duration(sa.Minute())*time.Minute +
+					time.Duration(sa.Second())*time.Second,
+			)
+		}
+		return nextRun{duration: sa.Sub(s.now()), dateTime: sa}
 	}
 
 	var next nextRun

scheduler_test.go

@@ -1894,6 +1894,20 @@ func TestScheduler_Update(t *testing.T) {
 
 		assert.Equal(t, expectedNext, actualNext)
 	})
+
+	// Verifies https://github.com/go-co-op/gocron/issues/499
+	t.Run("at time is not set when updating incompatible unit jobs", func(t *testing.T) {
+		s := NewScheduler(time.UTC)
+		startAt := time.Now().UTC().Add(time.Millisecond * 250)
+		j, err := s.Every(24 * time.Hour).StartAt(startAt).Do(func() {})
+		require.NoError(t, err)
+		s.StartAsync()
+		time.Sleep(time.Millisecond * 500)
+
+		_, err = s.Job(j).StartAt(startAt.Add(1 * time.Second)).Update()
+		require.NoError(t, err)
+		s.Stop()
+	})
 }
 
 func TestScheduler_RunByTag(t *testing.T) {