[BUG] - Time reported by NextRun when running WithSingletonMode and WithStartDateTime incorrect

[aneesh1]

Describe the bug

The time reported by NextRun isn't right when running WithSingletonMode and WithStartDateTime. For instance, suppose we have a job that starts at time t (in the future) and repeats with some interval i. When calling NextRun at some time t_i that is before t, the NextRun is reported correctly. However, after the job is run for the first time, the time reported by NextRun is t + 2i when it should be t + i instead. Here's a table depicting the pattern I observed

Interval when NextRun is called	Actual Return Value	Expected Return Value
0 -> t	t ✅	t
t -> t + i	t + 2i ❌	t + i
t + i -> t + 2i	t + 4i ❌	t + 2i

To Reproduce

Here's a test that I wrote in the job_test.go file:

func TestJob_NextRun(t *testing.T) {
	testTime := time.Now()

	s := newTestScheduler(t)

        // run a job every 10 milliseconds that starts 10 milliseconds after the current time
	j, err := s.NewJob(
		DurationJob(
			10*time.Millisecond,
		),
		NewTask(
			func() {},
		),
		WithStartAt(WithStartDateTime(testTime.Add(10*time.Millisecond))),
		WithSingletonMode(LimitModeReschedule),
	)
	require.NoError(t, err)

	s.Start()
	nextRun, err := j.NextRun()
	require.NoError(t, err)

        // `NextRun` should report `testTime.Add(10*time.Millisecond)`
	assert.Equal(t, testTime.Add(10*time.Millisecond), nextRun)

        // sleep for 11ms to to wait for the next job
	time.Sleep(11 * time.Millisecond)

	nextRun, err = j.NextRun()
	assert.NoError(t, err)

        // `NextRun` should report a time 20 milliseconds after `testTime`, but instead reports a value that is `30ms` after
	assert.Equal(t, testTime.Add(20*time.Millisecond), nextRun)
	assert.Equal(t, 20*time.Millisecond, nextRun.Sub(testTime))

	err = s.Shutdown()
	require.NoError(t, err)
}

Version

v2.2.9

Expected behavior

I expect that these two asserts:

	assert.Equal(t, testTime.Add(20*time.Millisecond), nextRun)
	assert.Equal(t, 20*time.Millisecond, nextRun.Sub(testTime))

should not fail. I also modified this test to see what happens when more jobs are run:

func TestJob_NextRun(t *testing.T) {
	testTime := time.Now()

	s := newTestScheduler(t)

        // run a job every 10 milliseconds that starts 10 milliseconds after the current time
	j, err := s.NewJob(
		DurationJob(
			10*time.Millisecond,
		),
		NewTask(
			func() {},
		),
		WithStartAt(WithStartDateTime(testTime.Add(10*time.Millisecond))),
		WithSingletonMode(LimitModeReschedule),
	)
	require.NoError(t, err)

	s.Start()
	nextRun, err := j.NextRun()
	require.NoError(t, err)

        // `NextRun` should report `testTime.Add(10*time.Millisecond)`
	assert.Equal(t, testTime.Add(10*time.Millisecond), nextRun)

        // sleep for 11ms to to wait for the next job
	time.Sleep(11 * time.Millisecond)

	nextRun, err = j.NextRun()
	assert.NoError(t, err)

        // `NextRun` should report a time 20 milliseconds after `testTime`, but instead reports a value that is `30ms` after
	assert.Equal(t, testTime.Add(20*time.Millisecond), nextRun)
	assert.Equal(t, 20*time.Millisecond, nextRun.Sub(testTime))

        // sleep for 11ms to to wait for the next job
	time.Sleep(11 * time.Millisecond)

	nextRun, err = j.NextRun()
	assert.NoError(t, err)

         // `NextRun` should report a time 30 milliseconds after `testTime`, but instead reports a value that is `50ms` after
	assert.Equal(t, testTime.Add(30*time.Millisecond), nextRun)
	assert.Equal(t, 30*time.Millisecond, nextRun.Sub(testTime))

	err = s.Shutdown()
	require.NoError(t, err)

Fortunately what is working: my job is running at the specified interval provided to DurationJob.

[aneesh1]

Further context: it looks gocron v2.2.1 reports correct values for lastRun and nextRun. However, the fix for #661 is not in this release.

[aneesh1]

Follow up question: when I disable WithSingletonMode the nextRun time is reported correctly - why?