time: add a fast path for the IMF-fixdate layout in AppendFormat

src/time/format.go

@@ -659,6 +659,8 @@ func (t Time) AppendFormat(b []byte, layout string) []byte {
 		return t.appendFormatRFC3339(b, false)
 	case RFC3339Nano:
 		return t.appendFormatRFC3339(b, true)
+	case imfFixdate:
+		return t.appendFormatIMFFixdate(b)
 	default:
 		return t.appendFormat(b, layout)
 	}

src/time/format_imffixdate.go

@@ -0,0 +1,41 @@
+// Copyright 2026 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package time
+
+// imfFixdate is the IMF-fixdate layout: the format HTTP senders are required
+// to generate for header fields carrying a date, defined in RFC 9110, section
+// 5.6.7 as a fixed-length, single-zone subset of the Internet Message Format
+// (RFC 5322). It is the value of net/http.TimeFormat, and after RFC 3339 it is
+// the most widely used machine-readable layout, which is why it is
+// specialized here.
+//
+// "GMT" is a literal in this layout, not a zone abbreviation, so formatting
+// does not convert. An HTTP-date represents an instant in UTC; supplying one
+// is the caller's responsibility, as net/http does.
+const imfFixdate = "Mon, 02 Jan 2006 15:04:05 GMT"
+
+// appendFormatIMFFixdate is a specialization of appendFormat for imfFixdate.
+// It formats t in its own location, exactly as the general path does.
+func (t Time) appendFormatIMFFixdate(b []byte) []byte {
+	_, _, abs := t.locabs()
+	days := abs.days()
+	year, month, day := days.date()
+	hour, min, sec := abs.clock()
+
+	b = append(b, days.weekday().String()[:3]...)
+	b = append(b, ',', ' ')
+	b = appendInt(b, day, 2)
+	b = append(b, ' ')
+	b = append(b, month.String()[:3]...)
+	b = append(b, ' ')
+	b = appendInt(b, year, 4)
+	b = append(b, ' ')
+	b = appendInt(b, hour, 2)
+	b = append(b, ':')
+	b = appendInt(b, min, 2)
+	b = append(b, ':')
+	b = appendInt(b, sec, 2)
+	return append(b, " GMT"...)
+}

src/time/format_test.go

@@ -1091,3 +1091,25 @@ func FuzzParseRFC3339(f *testing.F) {
 		}
 	})
 }
+
+// TestAppendFormatIMFFixdate checks the specialized formatter for the
+// IMF-fixdate layout (RFC 9110, section 5.6.7) against the general one, across
+// many instants and several locations. The non-UTC cases matter: "GMT" is a
+// literal in this layout, not a zone abbreviation, so the time must not be
+// converted.
+func TestAppendFormatIMFFixdate(t *testing.T) {
+	const imfFixdate = "Mon, 02 Jan 2006 15:04:05 GMT" // net/http.TimeFormat
+	locs := []*Location{UTC, Local, FixedZone("CEST", 2*60*60), FixedZone("X", -11*3600-1800)}
+	base := Date(1990, January, 1, 0, 0, 0, 0, UTC)
+	var got, want [64]byte
+	for _, loc := range locs {
+		for i := range 20000 {
+			tm := base.Add(Duration(i) * 7919 * Second).In(loc)
+			g := tm.AppendFormat(got[:0], imfFixdate)
+			w := AppendFormatAny(tm, want[:0], imfFixdate)
+			if !bytes.Equal(g, w) {
+				t.Fatalf("loc=%v t=%v AppendFormat=%q general=%q", loc, tm, g, w)
+			}
+		}
+	}
+}

src/time/time_test.go

@@ -1545,6 +1545,22 @@ func BenchmarkFormatRFC3339Nano(b *testing.B) {
 	}
 }
 
+func BenchmarkFormatIMFFixdate(b *testing.B) {
+	t := Unix(1265346057, 0)
+	for i := 0; i < b.N; i++ {
+		t.Format("Mon, 02 Jan 2006 15:04:05 GMT")
+	}
+}
+
+// BenchmarkFormatRFC1123 uses a layout the same length as the HTTP date
+// format.
+func BenchmarkFormatRFC1123(b *testing.B) {
+	t := Unix(1265346057, 0)
+	for i := 0; i < b.N; i++ {
+		t.Format("Mon, 02 Jan 2006 15:04:05 MST")
+	}
+}
+
 func BenchmarkFormatNow(b *testing.B) {
 	// Like BenchmarkFormat, but easier, because the time zone
 	// lookup cache is optimized for the present.