diff --git a/log/filelogger/log.go b/log/filelogger/log.go
index 789284276..f8c4da65b 100644
--- a/log/filelogger/log.go
+++ b/log/filelogger/log.go
@@ -106,6 +106,7 @@ func (w *logFileWriter) appendToFileLocked(out []byte) {
 	if w.fday != day {
 		w.startNewFileLocked()
 	}
+	out = removeDatePrefix(out)
 	if w.f != nil {
 		// RFC3339Nano but with a fixed number (3) of nanosecond digits:
 		const formatPre = "2006-01-02T15:04:05"
@@ -118,6 +119,30 @@ func (w *logFileWriter) appendToFileLocked(out []byte) {
 	}
 }
 
+func isNum(b byte) bool { return '0' <= b && b <= '9' }
+
+// removeDatePrefix returns a subslice of v with the log package's
+// standard datetime prefix format removed, if present.
+func removeDatePrefix(v []byte) []byte {
+	const format = "2009/01/23 01:23:23 "
+	if len(v) < len(format) {
+		return v
+	}
+	for i, b := range v[:len(format)] {
+		fb := format[i]
+		if isNum(fb) {
+			if !isNum(b) {
+				return v
+			}
+			continue
+		}
+		if b != fb {
+			return v
+		}
+	}
+	return v[len(format):]
+}
+
 // startNewFileLocked opens a new log file for writing
 // and also cleans up any old files.
 //
diff --git a/log/filelogger/log_test.go b/log/filelogger/log_test.go
new file mode 100644
index 000000000..74977292f
--- /dev/null
+++ b/log/filelogger/log_test.go
@@ -0,0 +1,28 @@
+// Copyright (c) 2021 Tailscale Inc & 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 filelogger
+
+import "testing"
+
+func TestRemoveDatePrefix(t *testing.T) {
+	tests := []struct {
+		in, want string
+	}{
+		{"", ""},
+		{"\n", "\n"},
+		{"2009/01/23 01:23:23", "2009/01/23 01:23:23"},
+		{"2009/01/23 01:23:23 \n", "\n"},
+		{"2009/01/23 01:23:23 foo\n", "foo\n"},
+		{"9999/01/23 01:23:23 foo\n", "foo\n"},
+		{"2009_01/23 01:23:23 had an underscore\n", "2009_01/23 01:23:23 had an underscore\n"},
+	}
+	for i, tt := range tests {
+		got := removeDatePrefix([]byte(tt.in))
+		if string(got) != tt.want {
+			t.Logf("[%d] removeDatePrefix(%q) = %q; want %q", i, tt.in, got, tt.want)
+		}
+	}
+
+}