diff --git a/pkg/isb/forward/forward.go b/pkg/isb/forward/forward.go
index e473dbac34..6483f68fc2 100644
--- a/pkg/isb/forward/forward.go
+++ b/pkg/isb/forward/forward.go
@@ -26,11 +26,12 @@ import (
 	"sync"
 	"time"
 
-	"github.com/numaproj/numaflow/pkg/watermark/fetch"
-	"github.com/numaproj/numaflow/pkg/watermark/publish"
 	"go.uber.org/zap"
 	"k8s.io/apimachinery/pkg/util/wait"
 
+	"github.com/numaproj/numaflow/pkg/watermark/fetch"
+	"github.com/numaproj/numaflow/pkg/watermark/publish"
+
 	dfv1 "github.com/numaproj/numaflow/pkg/apis/numaflow/v1alpha1"
 	"github.com/numaproj/numaflow/pkg/isb"
 	metricspkg "github.com/numaproj/numaflow/pkg/metrics"
@@ -194,8 +195,10 @@ func (isdf *InterStepDataForward) forwardAChunk(ctx context.Context) {
 
 	// fetch watermark if available
 	// TODO: make it async (concurrent and wait later)
-	// let's track only the last element's watermark
-	processorWM := isdf.fetchWatermark.GetWatermark(readMessages[len(readMessages)-1].ReadOffset)
+	// let's track only the first element's watermark. This is important because we reassign the watermark we fetch
+	// to all the elements in the batch. If we were to assign last element's watermark, we will wronly mark on-time data
+	// as πlate date.
+	processorWM := isdf.fetchWatermark.GetWatermark(readMessages[0].ReadOffset)
 	for _, m := range readMessages {
 		readBytesCount.With(map[string]string{metricspkg.LabelVertex: isdf.vertexName, metricspkg.LabelPipeline: isdf.pipelineName, "buffer": isdf.fromBuffer.GetName()}).Add(float64(len(m.Payload)))
 		m.Watermark = time.Time(processorWM)
diff --git a/pkg/sources/generator/tickgen.go b/pkg/sources/generator/tickgen.go
index f8a22b45d0..5810a50f57 100644
--- a/pkg/sources/generator/tickgen.go
+++ b/pkg/sources/generator/tickgen.go
@@ -244,7 +244,8 @@ loop:
 		// into the offset timeline store.
 		// Please note that we are inserting the watermark before the data has been persisted into ISB by the forwarder.
 		o := msgs[len(msgs)-1].ReadOffset
-		nanos, _ := o.Sequence()
+		// use the first eventime as watermark to make it conservative
+		nanos, _ := msgs[0].ReadOffset.Sequence()
 		// remove the nanosecond precision
 		mg.sourcePublishWM.PublishWatermark(processor.Watermark(time.Unix(0, nanos)), o)
 	}
diff --git a/pkg/sources/http/http.go b/pkg/sources/http/http.go
index aaa8a059c4..4ecea9dd50 100644
--- a/pkg/sources/http/http.go
+++ b/pkg/sources/http/http.go
@@ -217,14 +217,14 @@ func (h *httpSource) GetName() string {
 
 func (h *httpSource) Read(_ context.Context, count int64) ([]*isb.ReadMessage, error) {
 	msgs := []*isb.ReadMessage{}
-	var latest time.Time
+	var oldest time.Time
 	timeout := time.After(h.readTimeout)
 loop:
 	for i := int64(0); i < count; i++ {
 		select {
 		case m := <-h.messages:
-			if latest.IsZero() || m.EventTime.After(latest) {
-				latest = m.EventTime
+			if oldest.IsZero() || m.EventTime.Before(oldest) {
+				oldest = m.EventTime
 			}
 			msgs = append(msgs, m)
 			httpSourceReadCount.With(map[string]string{metricspkg.LabelVertex: h.name, metricspkg.LabelPipeline: h.pipelineName}).Inc()
@@ -234,8 +234,8 @@ loop:
 		}
 	}
 	h.logger.Debugf("Read %d messages.", len(msgs))
-	if len(msgs) > 0 && !latest.IsZero() {
-		h.sourcePublishWM.PublishWatermark(processor.Watermark(latest), msgs[len(msgs)-1].ReadOffset)
+	if len(msgs) > 0 && !oldest.IsZero() {
+		h.sourcePublishWM.PublishWatermark(processor.Watermark(oldest), msgs[len(msgs)-1].ReadOffset)
 	}
 	return msgs, nil
 }
diff --git a/pkg/sources/kafka/reader.go b/pkg/sources/kafka/reader.go
index 1deec3ba28..5fa31dd3a3 100644
--- a/pkg/sources/kafka/reader.go
+++ b/pkg/sources/kafka/reader.go
@@ -128,7 +128,7 @@ func (r *KafkaSource) GetName() string {
 // at-least-once semantics for reading, during restart we will have to reprocess all unacknowledged messages.
 func (r *KafkaSource) Read(_ context.Context, count int64) ([]*isb.ReadMessage, error) {
 	// It stores latest timestamps for different partitions
-	latestTimestamps := make(map[int32]time.Time)
+	oldestTimestamps := make(map[int32]time.Time)
 	msgs := make([]*isb.ReadMessage, 0, count)
 	timeout := time.After(r.readTimeout)
 loop:
@@ -139,8 +139,8 @@ loop:
 			_m := toReadMessage(m)
 			msgs = append(msgs, _m)
 			// Get latest timestamps for different partitions
-			if t, ok := latestTimestamps[m.Partition]; !ok || m.Timestamp.After(t) {
-				latestTimestamps[m.Partition] = m.Timestamp
+			if t, ok := oldestTimestamps[m.Partition]; !ok || m.Timestamp.Before(t) {
+				oldestTimestamps[m.Partition] = m.Timestamp
 			}
 		case <-timeout:
 			// log that timeout has happened and don't return an error
@@ -148,7 +148,7 @@ loop:
 			break loop
 		}
 	}
-	for p, t := range latestTimestamps {
+	for p, t := range oldestTimestamps {
 		publisher := r.loadSourceWartermarkPublisher(p)
 		publisher.PublishWatermark(processor.Watermark(t), nil) // Source publisher does not care about the offset
 	}