diff --git a/api/prometheus/v1/api.go b/api/prometheus/v1/api.go
index d7b5e8b5a..d43738b10 100644
--- a/api/prometheus/v1/api.go
+++ b/api/prometheus/v1/api.go
@@ -1575,6 +1575,50 @@ func (h *apiClientImpl) DoGetFallback(ctx context.Context, u *url.URL, args url.
 	return resp, body, warnings, err
 }
 
+var (
+	// Prometheus's HTTP API accepts these boundary timestamps explicitly even
+	// though Go's RFC3339 parser cannot round-trip years with more than four
+	// digits. Keep these values aligned with the server-side defaults so very
+	// low/high sentinels like model.Earliest and model.Latest map to something
+	// the API can parse without overflowing.
+	minTime = time.Unix(math.MinInt64/1000+62135596801, 0).UTC()
+	maxTime = time.Unix(math.MaxInt64/1000-62135596801, 999999999).UTC()
+
+	minTimeFormatted = minTime.Format(time.RFC3339Nano)
+	maxTimeFormatted = maxTime.Format(time.RFC3339Nano)
+)
+
 func formatTime(t time.Time) string {
-	return strconv.FormatFloat(float64(t.Unix())+float64(t.Nanosecond())/1e9, 'f', -1, 64)
+	switch {
+	case !t.After(minTime):
+		return minTimeFormatted
+	case !t.Before(maxTime):
+		return maxTimeFormatted
+	}
+
+	// Avoid t.UnixNano here: it is undefined for times that cannot be
+	// represented as an int64 nanosecond offset even though time.Unix and
+	// Nanosecond still expose the second/nanosecond components we need.
+	return formatTimeString(t.Unix(), t.Nanosecond())
+}
+
+func formatTimeString(sec int64, nsec int) string {
+	if sec >= 0 {
+		return formatTimeParts("", uint64(sec), nsec)
+	}
+	if nsec == 0 {
+		return strconv.FormatInt(sec, 10)
+	}
+
+	return formatTimeParts("-", uint64(-(sec + 1)), int(time.Second)-nsec)
+}
+
+func formatTimeParts(sign string, sec uint64, nsec int) string {
+	s := sign + strconv.FormatUint(sec, 10)
+	if nsec == 0 {
+		return s
+	}
+
+	fraction := strconv.Itoa(nsec + int(time.Second))[1:]
+	return s + "." + strings.TrimRight(fraction, "0")
 }
diff --git a/api/prometheus/v1/api_test.go b/api/prometheus/v1/api_test.go
index f846bd9b5..1c5e7b8fc 100644
--- a/api/prometheus/v1/api_test.go
+++ b/api/prometheus/v1/api_test.go
@@ -1613,6 +1613,43 @@ func TestAPIClientDo(t *testing.T) {
 	}
 }
 
+func TestFormatTimeUsesModelTimestampEncoding(t *testing.T) {
+	tests := []struct {
+		name string
+		ts   time.Time
+		want string
+	}{
+		{
+			name: "regular time keeps millisecond precision",
+			ts:   time.Unix(1700000000, 123456789).UTC(),
+			want: "1700000000.123456789",
+		},
+		{
+			name: "negative fractional times keep the correct sign and fraction",
+			ts:   time.Unix(-1, 100000000).UTC(),
+			want: "-0.9",
+		},
+		{
+			name: "model earliest clamps to prometheus parseable boundary",
+			ts:   model.Earliest.Time(),
+			want: minTimeFormatted,
+		},
+		{
+			name: "model latest clamps to prometheus parseable boundary",
+			ts:   model.Latest.Time(),
+			want: maxTimeFormatted,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := formatTime(tt.ts); got != tt.want {
+				t.Fatalf("formatTime() = %q, want %q", got, tt.want)
+			}
+		})
+	}
+}
+
 func TestSamplesJSONSerialization(t *testing.T) {
 	tests := []struct {
 		point    model.SamplePair