diff --git a/pkg/servers/aggregator/aggregator.go b/pkg/servers/aggregator/aggregator.go
index eb63d7f..1e53648 100644
--- a/pkg/servers/aggregator/aggregator.go
+++ b/pkg/servers/aggregator/aggregator.go
@@ -8,7 +8,7 @@ import (
 )
 
 const (
-	AggregationWindowSeconds = 10
+	aggregationWindow = 10 * time.Second
 )
 
 type Key struct {
@@ -21,11 +21,12 @@ type Key struct {
 }
 
 type Aggregator struct {
-	data                   map[Key]*flow.Flow
-	stopCh                 chan struct{}
-	ingress                chan *flow.Flow
-	output                 chan []*flow.Flow
-	currentUnixTimeSeconds int64
+	data      map[Key]*flow.Flow
+	stopCh    chan struct{}
+	ingress   chan *flow.Flow
+	output    chan []*flow.Flow
+	lastFlush time.Time
+	timeNow   func() time.Time
 }
 
 func New(output chan []*flow.Flow) *Aggregator {
@@ -34,6 +35,7 @@ func New(output chan []*flow.Flow) *Aggregator {
 		stopCh:  make(chan struct{}),
 		ingress: make(chan *flow.Flow),
 		output:  output,
+		timeNow: time.Now,
 	}
 
 	go a.service()
@@ -77,15 +79,15 @@ func (a *Aggregator) service() {
 }
 
 func (a *Aggregator) Ingest(fl *flow.Flow) {
-	currentUnixTimeSeconds := time.Now().Unix()
-	currentUnixTimeSeconds -= currentUnixTimeSeconds % AggregationWindowSeconds
+	normalizedIngestTime := a.timeNow().Truncate(aggregationWindow)
 
-	if a.currentUnixTimeSeconds < currentUnixTimeSeconds {
+	timeSinceLastFlush := normalizedIngestTime.Sub(a.lastFlush)
+	if timeSinceLastFlush >= aggregationWindow {
 		a.flush()
-		a.currentUnixTimeSeconds = currentUnixTimeSeconds
+		a.lastFlush = normalizedIngestTime
 	}
 
-	fl.Timestamp = currentUnixTimeSeconds
+	fl.Timestamp = normalizedIngestTime.Unix()
 	a.add(fl)
 }
 
diff --git a/pkg/servers/aggregator/aggregator_test.go b/pkg/servers/aggregator/aggregator_test.go
new file mode 100644
index 0000000..b38399f
--- /dev/null
+++ b/pkg/servers/aggregator/aggregator_test.go
@@ -0,0 +1,81 @@
+package aggregator
+
+import (
+	"testing"
+	"time"
+
+	"github.com/bio-routing/bio-rd/net"
+	"github.com/bio-routing/flowhouse/pkg/models/flow"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func exampleFlow(t testing.TB, ts time.Time) *flow.Flow {
+	return &flow.Flow{
+		Agent:     must[net.IP](t)(net.IPFromString("2001:db8::1")),
+		SrcPort:   34567,
+		DstPort:   443,
+		Packets:   10,
+		Protocol:  6,
+		Family:    4,
+		Timestamp: ts.Unix(),
+		Size:      200,
+		SrcAddr:   must[net.IP](t)(net.IPFromString("198.51.100.24")),
+		DstAddr:   must[net.IP](t)(net.IPFromString("203.0.113.30")),
+	}
+}
+
+func TestAggregatorBuffering(t *testing.T) {
+	// align initial time to avoid test flakiness
+	initialTime := time.Now().Truncate(10 * time.Second)
+	mockedTime := initialTime
+
+	out := make(chan []*flow.Flow, 10)
+	agg := New(out)
+	agg.timeNow = func() time.Time { return mockedTime }
+
+	agg.Ingest(exampleFlow(t, mockedTime))
+	// should have flushed an empty list at the beginning
+	select {
+	case flows := <-out:
+		assert.Empty(t, flows)
+	default:
+		t.Error("no flows in channel")
+	}
+
+	// advance time by 2 seconds
+	mockedTime = mockedTime.Add(2 * time.Second)
+
+	agg.Ingest(exampleFlow(t, mockedTime))
+	assert.Len(t, out, 0) // should not have flushed
+
+	// advance time by 10 seconds
+	mockedTime = mockedTime.Add(10 * time.Second)
+
+	agg.Ingest(exampleFlow(t, mockedTime))
+	assert.Len(t, out, 1) // should have flushed once
+
+	// check flushed flows
+	{
+		select {
+		case flows := <-out:
+			require.Len(t, flows, 1)
+			fl := flows[0]
+			assert.Equal(t,
+				initialTime.Truncate(10*time.Second).Unix(),
+				fl.Timestamp,
+			)
+		default:
+			t.Error("no flow available")
+		}
+	}
+}
+
+func must[T any](t testing.TB) func(res T, err error) T {
+	return func(res T, err error) T {
+		if err != nil {
+			t.Error(err)
+		}
+		return res
+	}
+}