Show the last event as a relative time with a UTC tooltip (closes #567)

The "Last event" figure in the webhook page's Statistics block now shows how long ago the newest event arrived, as its two largest units of d, h, m and s with no spaces ("59s", "1m1s", "3d4h"; a zero second unit is left off). It is worked out when the page is served, and a time ahead of the server clock shows "0s". Its tooltip is the full time in UTC. With no event it still shows "none", without a tooltip. A unit test pins each unit boundary, and the headless-browser test checks the figure and its tooltip.

Model: opus-5-5

internal/handlers/export_test.go

@@ -96,6 +96,12 @@ func (s *Handlers) WebhookStatsForTest(webhookID string) *WebhookStats {
 	return s.loadWebhookStats(webhookID, entrypoints, targets)
 }
 
+// RelativeTimeForTest exposes relativeTime for use in the
+// handlers_test package.
+func RelativeTimeForTest(at, now time.Time) string {
+	return relativeTime(at, now)
+}
+
 // FinishedByTargetForTest exposes finishedByTarget for use in the
 // handlers_test package.
 func FinishedByTargetForTest(

internal/handlers/webhook_stats.go

@@ -15,6 +15,9 @@ const (
 	longWindow  = 24 * time.Hour
 )
 
+// day is the largest unit relativeTime writes.
+const day = 24 * time.Hour
+
 // percent turns a fraction into a percentage.
 const percent = 100
 
@@ -35,8 +38,12 @@ type WebhookStats struct {
 	InProgress int64
 
 	// LastEventAt is when the newest event arrived, or nil when none
-	// has. Retention does not change it.
-	LastEventAt *time.Time
+	// has. Retention does not change it. LastEvent is how long ago
+	// that was, as relativeTime writes it, and LastEventUTC the full
+	// time; both are empty when no event has arrived.
+	LastEventAt  *time.Time
+	LastEvent    string
+	LastEventUTC string
 
 	Last10Minutes RecentWindow
 	Last24Hours   RecentWindow
@@ -81,6 +88,42 @@ func (w RecentWindow) FailurePercent() string {
 	)
 }
 
+// relativeTime is how long before now the time at was, in whole
+// seconds, written as its two largest units out of d, h, m and s:
+// "59s", "1m1s", "23h59m", "3d4h". A second unit of zero is left off,
+// as in "5m". A time after now is "0s".
+func relativeTime(at, now time.Time) string {
+	elapsed := max(now.Sub(at), 0)
+
+	days := int64(elapsed / day)
+	hours := int64(elapsed % day / time.Hour)
+	minutes := int64(elapsed % time.Hour / time.Minute)
+	seconds := int64(elapsed % time.Minute / time.Second)
+
+	switch {
+	case days > 0:
+		return twoUnits(days, "d", hours, "h")
+	case hours > 0:
+		return twoUnits(hours, "h", minutes, "m")
+	case minutes > 0:
+		return twoUnits(minutes, "m", seconds, "s")
+	default:
+		return fmt.Sprintf("%ds", seconds)
+	}
+}
+
+// twoUnits writes a count of a larger unit followed by a count of the
+// next smaller one, leaving the smaller off when it is zero.
+func twoUnits(
+	larger int64, largerUnit string, smaller int64, smallerUnit string,
+) string {
+	if smaller == 0 {
+		return fmt.Sprintf("%d%s", larger, largerUnit)
+	}
+
+	return fmt.Sprintf("%d%s%d%s", larger, largerUnit, smaller, smallerUnit)
+}
+
 // loadWebhookStats gathers the figures for the statistics pane from the
 // webhook's entrypoints and targets, as the page has already loaded
 // them, and from its event database. It returns nil, and logs why, when
@@ -139,7 +182,7 @@ func (h *Handlers) loadWebhookStats(
 func readEventStats(
 	db *gorm.DB, now time.Time, stats *WebhookStats,
 ) error {
-	err := readTotals(db, stats)
+	err := readTotals(db, now, stats)
 	if err != nil {
 		return err
 	}
@@ -169,9 +212,9 @@ func readEventStats(
 }
 
 // readTotals fills in the lifetime and within-retention figures, and
-// when the last event arrived, from the running totals: the events'
-// row, and the targets' rows summed.
-func readTotals(db *gorm.DB, stats *WebhookStats) error {
+// when the last event arrived and how long before now that was, from
+// the running totals: the events' row, and the targets' rows summed.
+func readTotals(db *gorm.DB, now time.Time, stats *WebhookStats) error {
 	var events database.EventTotals
 
 	err := db.Take(&events).Error
@@ -190,6 +233,12 @@ func readTotals(db *gorm.DB, stats *WebhookStats) error {
 	stats.WithinRetention.Events = events.Events - events.EventsRemoved
 	stats.LastEventAt = events.LastEventAt
 
+	if events.LastEventAt != nil {
+		stats.LastEvent = relativeTime(*events.LastEventAt, now)
+		stats.LastEventUTC =
+			events.LastEventAt.UTC().Format(time.DateTime) + " UTC"
+	}
+
 	for _, t := range targets {
 		stats.Lifetime.Deliveries += t.Deliveries
 		stats.Lifetime.Failures += t.Failed

internal/handlers/webhook_stats_test.go

@@ -265,6 +265,13 @@ func statsPane(t *testing.T, page string) string {
 	return strings.Join(strings.Fields(pane), " ")
 }
 
+// statsLastEventShown matches the pane's last event for an event that
+// arrived at at: a figure with the full time in UTC as its tooltip.
+func statsLastEventShown(at time.Time) string {
+	return `Last event</span>\s*<span [^>]*title="` +
+		at.UTC().Format(time.DateTime) + ` UTC">[^<]+</span>`
+}
+
 // assertStatsTargets checks, for the history seedStatsHistory builds,
 // each target's totals and its deliveries finished in the last 24
 // hours. The first target has ten deliveries and the replay, the
@@ -300,14 +307,17 @@ func assertStatsPaneAfterPrune(
 ) {
 	t.Helper()
 
-	pane := statsPane(t, renderSourceDetailPage(t, h, sess, hist.webhook.ID))
-	lastEvent := hist.newest.CreatedAt.UTC().Format("2006-01-02 15:04:05 UTC")
+	page := renderSourceDetailPage(t, h, sess, hist.webhook.ID)
+	pane := statsPane(t, page)
 
-	assert.Contains(t, pane, "Entrypoints 12 (11 active) "+
-		"Targets 6 (2 active) "+
-		"Deliveries in progress 13 "+
-		"Last event "+lastEvent+" "+
-		"Retention 14 days")
+	// The newest event arrived moments ago, so the last event is a
+	// few seconds ago.
+	assert.Regexp(t, `Entrypoints 12 \(11 active\) `+
+		`Targets 6 \(2 active\) `+
+		`Deliveries in progress 13 `+
+		`Last event \d+s `+
+		`Retention 14 days`, pane)
+	assert.Regexp(t, statsLastEventShown(hist.newest.CreatedAt), page)
 	assert.Contains(t, pane, "Lifetime Within retention "+
 		"Events 10 9 "+
 		"Deliveries 21 18 "+
@@ -440,15 +450,14 @@ func TestWebhookStats_LastEventSurvivesPruningEveryEvent(t *testing.T) {
 	require.NotNil(t, stats.LastEventAt)
 	assert.True(t, arrived.Equal(*stats.LastEventAt))
 
-	pane := statsPane(t, renderSourceDetailPage(t, h, sess, wh.ID))
-	assert.Contains(t, pane,
-		"Last event "+arrived.UTC().Format("2006-01-02 15:04:05 UTC"))
+	assert.Regexp(t, statsLastEventShown(arrived),
+		renderSourceDetailPage(t, h, sess, wh.ID))
 }
 
-// TestWebhookStats_LastEventInUTC checks that the pane shows when the
-// last event arrived in UTC, as the event list does, when the time was
-// stored in another zone, as it is on a host whose local time is not
-// UTC.
+// TestWebhookStats_LastEventInUTC checks that the tooltip of the pane's
+// last event gives the time in UTC, as the event list does, when the
+// time was stored in another zone, as it is on a host whose local time
+// is not UTC.
 func TestWebhookStats_LastEventInUTC(t *testing.T) {
 	t.Parallel()
 
@@ -474,8 +483,9 @@ func TestWebhookStats_LastEventInUTC(t *testing.T) {
 	require.NoError(t, database.AddEventTotals(webhookDB,
 		database.EventTotals{Events: 1, LastEventAt: &arrived}))
 
-	pane := statsPane(t, renderSourceDetailPage(t, h, sess, wh.ID))
-	assert.Contains(t, pane, "Last event 2026-03-05 03:30:00 UTC")
+	assert.Regexp(t,
+		`Last event</span>\s*<span [^>]*title="2026-03-05 03:30:00 UTC">`,
+		renderSourceDetailPage(t, h, sess, wh.ID))
 }
 
 // TestWebhookStats_PaneShowsRetentionPeriod checks that the statistics
@@ -541,8 +551,11 @@ func TestWebhookStats_WebhookWithNoEvents(t *testing.T) {
 	assert.Equal(t, &handlers.WebhookStats{}, h.WebhookStatsForTest(wh.ID))
 	assert.Equal(t, "—", handlers.RecentWindow{}.FailurePercent())
 
-	pane := statsPane(t, renderSourceDetailPage(t, h, sess, wh.ID))
+	page := renderSourceDetailPage(t, h, sess, wh.ID)
+	pane := statsPane(t, page)
 	assert.Contains(t, pane, "Last event none")
+	assert.Regexp(t, `Last event</span>\s*<span class="[^"]*">none</span>`,
+		page, "a last event of none has a tooltip")
 	assert.Contains(t, pane, "Failure percentage — —")
 	assert.False(t, dbMgr.DBExists(wh.ID))
 }
@@ -567,3 +580,35 @@ func TestRecentWindow_FailurePercent(t *testing.T) {
 		assert.Equal(t, tt.want, tt.window.FailurePercent(), tt.window)
 	}
 }
+
+// TestRelativeTime pins how the pane writes the time since the last
+// event, at each boundary between its units.
+func TestRelativeTime(t *testing.T) {
+	t.Parallel()
+
+	now := time.Date(2026, time.October, 26, 12, 0, 0, 0, time.UTC)
+
+	tests := []struct {
+		ago  time.Duration
+		want string
+	}{
+		{0, "0s"},
+		{59900 * time.Millisecond, "59s"},
+		{60 * time.Second, "1m"},
+		{61 * time.Second, "1m1s"},
+		{3599 * time.Second, "59m59s"},
+		{3600 * time.Second, "1h"},
+		{3660 * time.Second, "1h1m"},
+		{86399 * time.Second, "23h59m"},
+		{86400 * time.Second, "1d"},
+		{90000 * time.Second, "1d1h"},
+		{400*24*time.Hour + 3*time.Hour, "400d3h"},
+		// An event time ahead of the server clock.
+		{-90 * time.Second, "0s"},
+	}
+
+	for _, tt := range tests {
+		assert.Equal(t, tt.want,
+			handlers.RelativeTimeForTest(now.Add(-tt.ago), now), tt.ago)
+	}
+}