feat: add WithStartDateTimePast WithStartAt option (#882)

errors.go

@@ -54,6 +54,7 @@ var (
 	ErrWithMonitorNil                = errors.New("gocron: WithMonitor: monitor must not be nil")
 	ErrWithNameEmpty                 = errors.New("gocron: WithName: name must not be empty")
 	ErrWithStartDateTimePast         = errors.New("gocron: WithStartDateTime: start must not be in the past")
+	ErrWithStartDateTimePastZero     = errors.New("gocron: WithStartDateTime: start must not be zero")
 	ErrWithStopDateTimePast          = errors.New("gocron: WithStopDateTime: end must not be in the past")
 	ErrStartTimeLaterThanEndTime     = errors.New("gocron: WithStartDateTime: start must not be later than end")
 	ErrStopTimeEarlierThanStartTime  = errors.New("gocron: WithStopDateTime: end must not be earlier than start")

example_test.go

@@ -1046,6 +1046,90 @@ func ExampleWithStartAt() {
 	// 9999-09-09 09:09:09.000000009 +0000 UTC
 }
 
+func ExampleWithStartDateTime() {
+	s, _ := gocron.NewScheduler()
+	defer func() { _ = s.Shutdown() }()
+
+	start := time.Date(9999, 9, 9, 9, 9, 9, 9, time.UTC)
+
+	j, _ := s.NewJob(
+		gocron.DurationJob(
+			time.Second,
+		),
+		gocron.NewTask(
+			func(one string, two int) {
+				fmt.Printf("%s, %d", one, two)
+			},
+			"one", 2,
+		),
+		gocron.WithStartAt(
+			gocron.WithStartDateTime(start),
+		),
+	)
+	s.Start()
+
+	next, _ := j.NextRun()
+	fmt.Println(next)
+
+	_ = s.StopJobs()
+	// Output:
+	// 9999-09-09 09:09:09.000000009 +0000 UTC
+}
+
+func ExampleWithStartDateTimePast() {
+	s, _ := gocron.NewScheduler()
+	defer func() { _ = s.Shutdown() }()
+
+	start := time.Now().Add(-time.Minute)
+
+	j, _ := s.NewJob(
+		gocron.DurationJob(
+			time.Second,
+		),
+		gocron.NewTask(
+			func(one string, two int) {
+				fmt.Printf("%s, %d", one, two)
+			},
+			"one", 2,
+		),
+		gocron.WithStartAt(
+			gocron.WithStartDateTimePast(start),
+		),
+	)
+	s.Start()
+
+	time.Sleep(100 * time.Millisecond)
+
+	_, _ = j.NextRun()
+
+	_ = s.StopJobs()
+}
+
+func ExampleWithStartImmediately() {
+	s, _ := gocron.NewScheduler()
+	defer func() { _ = s.Shutdown() }()
+
+	j, _ := s.NewJob(
+		gocron.DurationJob(
+			time.Second,
+		),
+		gocron.NewTask(
+			func(one string, two int) {
+				fmt.Printf("%s, %d", one, two)
+			},
+			"one", 2,
+		),
+		gocron.WithStartAt(
+			gocron.WithStartImmediately(),
+		),
+	)
+	s.Start()
+
+	_, _ = j.NextRun()
+
+	_ = s.StopJobs()
+}
+
 func ExampleWithStopTimeout() {
 	_, _ = gocron.NewScheduler(
 		gocron.WithStopTimeout(time.Second * 5),

job.go

@@ -718,6 +718,26 @@ func WithStartDateTime(start time.Time) StartAtOption {
 	}
 }
 
+// WithStartDateTimePast sets the first date & time at which the job should run
+// from a time in the past. This is useful when you want to backdate
+// the start time of a job to a time in the past, for example
+// if you want to start a job from a specific date in the past
+// and have it run on its schedule from then.
+// The start time can be in the past, but not zero.
+// If the start time is in the future, it behaves the same as WithStartDateTime.
+func WithStartDateTimePast(start time.Time) StartAtOption {
+	return func(j *internalJob, _ time.Time) error {
+		if start.IsZero() {
+			return ErrWithStartDateTimePastZero
+		}
+		if !j.stopTime.IsZero() && j.stopTime.Before(start) {
+			return ErrStartTimeLaterThanEndTime
+		}
+		j.startTime = start
+		return nil
+	}
+}
+
 // WithStopAt sets the option for stopping the job from running
 // after the specified time.
 func WithStopAt(option StopAtOption) JobOption {

scheduler_test.go

@@ -1004,6 +1004,14 @@ func TestScheduler_NewJobErrors(t *testing.T) {
 			[]JobOption{WithStartAt(WithStartDateTime(time.Now().Add(-time.Second)))},
 			ErrWithStartDateTimePast,
 		},
+		{
+			"WithStartDateTimePast is zero",
+			DurationJob(
+				time.Second,
+			),
+			[]JobOption{WithStartAt(WithStartDateTimePast(time.Time{}))},
+			ErrWithStartDateTimePastZero,
+		},
 		{
 			"WithStartDateTime is later than the end",
 			DurationJob(