diff --git a/src/pkg/archive/tar/tar_test.go b/src/pkg/archive/tar/tar_test.go
index 0adc179003..a509f3c00a 100644
--- a/src/pkg/archive/tar/tar_test.go
+++ b/src/pkg/archive/tar/tar_test.go
@@ -5,7 +5,10 @@
 package tar
 
 import (
+	"bytes"
+	"io/ioutil"
 	"os"
+	"reflect"
 	"testing"
 	"time"
 )
@@ -54,3 +57,43 @@ func (symlink) Mode() os.FileMode  { return os.ModeSymlink }
 func (symlink) ModTime() time.Time { return time.Time{} }
 func (symlink) IsDir() bool        { return false }
 func (symlink) Sys() interface{}   { return nil }
+
+func TestRoundTrip(t *testing.T) {
+	data := []byte("some file contents")
+
+	var b bytes.Buffer
+	tw := NewWriter(&b)
+	hdr := &Header{
+		Name:    "file.txt",
+		Size:    int64(len(data)),
+		ModTime: time.Now(),
+	}
+	// tar only supports second precision.
+	hdr.ModTime = hdr.ModTime.Add(-time.Duration(hdr.ModTime.Nanosecond()) * time.Nanosecond)
+	if err := tw.WriteHeader(hdr); err != nil {
+		t.Fatalf("tw.WriteHeader: %v", err)
+	}
+	if _, err := tw.Write(data); err != nil {
+		t.Fatalf("tw.Write: %v", err)
+	}
+	if err := tw.Close(); err != nil {
+		t.Fatalf("tw.Close: %v", err)
+	}
+
+	// Read it back.
+	tr := NewReader(&b)
+	rHdr, err := tr.Next()
+	if err != nil {
+		t.Fatalf("tr.Next: %v", err)
+	}
+	if !reflect.DeepEqual(rHdr, hdr) {
+		t.Errorf("Header mismatch.\n got %+v\nwant %+v", rHdr, hdr)
+	}
+	rData, err := ioutil.ReadAll(tr)
+	if err != nil {
+		t.Fatalf("Read: %v", err)
+	}
+	if !bytes.Equal(rData, data) {
+		t.Errorf("Data mismatch.\n got %q\nwant %q", rData, data)
+	}
+}
diff --git a/src/pkg/archive/tar/writer.go b/src/pkg/archive/tar/writer.go
index a9c8fdbbc9..5af504b437 100644
--- a/src/pkg/archive/tar/writer.go
+++ b/src/pkg/archive/tar/writer.go
@@ -12,6 +12,7 @@ import (
 	"fmt"
 	"io"
 	"strconv"
+	"time"
 )
 
 var (
@@ -110,6 +111,12 @@ func (tw *Writer) numeric(b []byte, x int64) {
 	b[0] |= 0x80 // highest bit indicates binary format
 }
 
+var (
+	minTime = time.Unix(0, 0)
+	// There is room for 11 octal digits (33 bits) of mtime.
+	maxTime = minTime.Add((1<<33 - 1) * time.Second)
+)
+
 // WriteHeader writes hdr and prepares to accept the file's contents.
 // WriteHeader calls Flush if it is not the first header.
 // Calling after a Close will return ErrWriteAfterClose.
@@ -133,19 +140,25 @@ func (tw *Writer) WriteHeader(hdr *Header) error {
 	// TODO(dsymonds): handle names longer than 100 chars
 	copy(s.next(100), []byte(hdr.Name))
 
-	tw.octal(s.next(8), hdr.Mode)              // 100:108
-	tw.numeric(s.next(8), int64(hdr.Uid))      // 108:116
-	tw.numeric(s.next(8), int64(hdr.Gid))      // 116:124
-	tw.numeric(s.next(12), hdr.Size)           // 124:136
-	tw.numeric(s.next(12), hdr.ModTime.Unix()) // 136:148
-	s.next(8)                                  // chksum (148:156)
-	s.next(1)[0] = hdr.Typeflag                // 156:157
-	tw.cString(s.next(100), hdr.Linkname)      // linkname (157:257)
-	copy(s.next(8), []byte("ustar\x0000"))     // 257:265
-	tw.cString(s.next(32), hdr.Uname)          // 265:297
-	tw.cString(s.next(32), hdr.Gname)          // 297:329
-	tw.numeric(s.next(8), hdr.Devmajor)        // 329:337
-	tw.numeric(s.next(8), hdr.Devminor)        // 337:345
+	// Handle out of range ModTime carefully.
+	var modTime int64
+	if !hdr.ModTime.Before(minTime) && !hdr.ModTime.After(maxTime) {
+		modTime = hdr.ModTime.Unix()
+	}
+
+	tw.octal(s.next(8), hdr.Mode)          // 100:108
+	tw.numeric(s.next(8), int64(hdr.Uid))  // 108:116
+	tw.numeric(s.next(8), int64(hdr.Gid))  // 116:124
+	tw.numeric(s.next(12), hdr.Size)       // 124:136
+	tw.numeric(s.next(12), modTime)        // 136:148
+	s.next(8)                              // chksum (148:156)
+	s.next(1)[0] = hdr.Typeflag            // 156:157
+	tw.cString(s.next(100), hdr.Linkname)  // linkname (157:257)
+	copy(s.next(8), []byte("ustar\x0000")) // 257:265
+	tw.cString(s.next(32), hdr.Uname)      // 265:297
+	tw.cString(s.next(32), hdr.Gname)      // 297:329
+	tw.numeric(s.next(8), hdr.Devmajor)    // 329:337
+	tw.numeric(s.next(8), hdr.Devminor)    // 337:345
 
 	// Use the GNU magic instead of POSIX magic if we used any GNU extensions.
 	if tw.usedBinary {