diff --git a/pkg/runner/aggregate_sender.go b/pkg/runner/aggregate_sender.go
index 440835e..03c4de4 100644
--- a/pkg/runner/aggregate_sender.go
+++ b/pkg/runner/aggregate_sender.go
@@ -8,7 +8,6 @@ import (
 	"fmt"
 	"io"
 	"log/slog"
-	"math"
 	"net"
 	"net/http"
 	"net/url"
@@ -126,10 +125,8 @@ func (as aggregateSender) send(fileName string, ts time.Time, duration time.Dura
 	req.ContentLength = fileSize
 
 	// Expected by aggrec, e.g:
-	// Aggregate-Interval: 2023-11-16T09:24:13.487591+01:00/PT1M
-	minutesFloat := duration.Minutes()
-	minutes := int(math.Round(minutesFloat))
-	req.Header.Add("Aggregate-Interval", fmt.Sprintf("%s/PT%dM", ts.Truncate(time.Minute).Format(time.RFC3339), minutes))
+	// Aggregate-Interval: 2023-11-16T09:24:13+01:00/PT45S
+	req.Header.Add("Aggregate-Interval", fmt.Sprintf("%s/%s", ts.Format(time.RFC3339), iso8601Duration(duration)))
 
 	as.log.Info("aggregateSender.send", "filename", fileName, "url", histogramURL)
 	startTime := time.Now()
@@ -171,3 +168,36 @@ func (as aggregateSender) send(fileName string, ts time.Time, duration time.Dura
 
 	return nil
 }
+
+func iso8601Duration(duration time.Duration) string {
+	if duration <= 0 {
+		return "PT0S"
+	}
+
+	total := int64(duration / time.Second)
+	nanoseconds := duration % time.Second
+
+	hours := total / 3600
+	total %= 3600
+	minutes := total / 60
+	seconds := total % 60
+
+	res := "PT"
+	if hours > 0 {
+		res += strconv.FormatInt(hours, 10) + "H"
+	}
+	if minutes > 0 {
+		res += strconv.FormatInt(minutes, 10) + "M"
+	}
+	if seconds > 0 || nanoseconds > 0 || res == "PT" {
+		if nanoseconds == 0 {
+			res += strconv.FormatInt(seconds, 10)
+		} else {
+			secondsFloat := float64(seconds) + float64(nanoseconds)/float64(time.Second)
+			res += strconv.FormatFloat(secondsFloat, 'f', -1, 64)
+		}
+		res += "S"
+	}
+
+	return res
+}
diff --git a/pkg/runner/aggregate_sender_test.go b/pkg/runner/aggregate_sender_test.go
new file mode 100644
index 0000000..732e1e4
--- /dev/null
+++ b/pkg/runner/aggregate_sender_test.go
@@ -0,0 +1,127 @@
+package runner
+
+import (
+	"crypto/ed25519"
+	"crypto/rand"
+	"io"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"os"
+	"testing"
+	"time"
+
+	"github.com/lestrrat-go/jwx/v2/jwa"
+	"github.com/lestrrat-go/jwx/v2/jwk"
+)
+
+func TestISO8601Duration(t *testing.T) {
+	tests := []struct {
+		name string
+		in   time.Duration
+		want string
+	}{
+		{
+			name: "zero",
+			in:   0,
+			want: "PT0S",
+		},
+		{
+			name: "seconds",
+			in:   45 * time.Second,
+			want: "PT45S",
+		},
+		{
+			name: "minutes",
+			in:   time.Minute,
+			want: "PT1M",
+		},
+		{
+			name: "mixed",
+			in:   time.Hour + 2*time.Minute + 3*time.Second,
+			want: "PT1H2M3S",
+		},
+		{
+			name: "fractional seconds",
+			in:   1500 * time.Millisecond,
+			want: "PT1.5S",
+		},
+		{
+			name: "negative clamps to zero",
+			in:   -time.Second,
+			want: "PT0S",
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := iso8601Duration(tc.in); got != tc.want {
+				t.Fatalf("have: %s, want: %s", got, tc.want)
+			}
+		})
+	}
+}
+
+func TestAggregateSenderUsesExactIntervalHeader(t *testing.T) {
+	var gotInterval string
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		gotInterval = r.Header.Get("Aggregate-Interval")
+		if _, err := io.Copy(io.Discard, r.Body); err != nil {
+			t.Fatalf("reading request body: %s", err)
+		}
+		w.Header().Set("Location", "/stored")
+		w.WriteHeader(http.StatusCreated)
+	}))
+	t.Cleanup(server.Close)
+
+	file, err := os.CreateTemp(t.TempDir(), "aggregate-*.parquet")
+	if err != nil {
+		t.Fatalf("CreateTemp: %s", err)
+	}
+	if _, err := file.WriteString("payload"); err != nil {
+		t.Fatalf("write temp aggregate: %s", err)
+	}
+	if err := file.Close(); err != nil {
+		t.Fatalf("close temp aggregate: %s", err)
+	}
+
+	_, signingKey, err := ed25519.GenerateKey(rand.Reader)
+	if err != nil {
+		t.Fatalf("GenerateKey: %s", err)
+	}
+	signingJWK, err := jwk.FromRaw(signingKey)
+	if err != nil {
+		t.Fatalf("FromRaw: %s", err)
+	}
+	if err := signingJWK.Set(jwk.AlgorithmKey, jwa.EdDSA); err != nil {
+		t.Fatalf("set Algorithm: %s", err)
+	}
+	if err := signingJWK.Set(jwk.KeyIDKey, "aggregate-test-key"); err != nil {
+		t.Fatalf("set KeyID: %s", err)
+	}
+
+	aggrecURL, err := url.Parse(server.URL)
+	if err != nil {
+		t.Fatalf("parse server URL: %s", err)
+	}
+
+	edm := &dnstapMinimiser{
+		log:                 slog.New(slog.NewTextHandler(io.Discard, nil)),
+		httpClientCertStore: newCertStore(),
+	}
+	as, err := edm.newAggregateSender(aggrecURL, signingJWK, nil)
+	if err != nil {
+		t.Fatalf("newAggregateSender: %s", err)
+	}
+
+	start := time.Date(2026, 4, 29, 12, 34, 45, 0, time.UTC)
+	if err := as.send(file.Name(), start, 45*time.Second); err != nil {
+		t.Fatalf("send: %s", err)
+	}
+
+	want := "2026-04-29T12:34:45Z/PT45S"
+	if gotInterval != want {
+		t.Fatalf("Aggregate-Interval header\n  have: %s\n  want: %s", gotInterval, want)
+	}
+}