diff --git a/tool/check-copyright-dates b/tool/check-copyright-dates
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+++ b/tool/check-copyright-dates
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+#!/bin/sh
+# tool/check-copyright-dates -- check copyrights dates are consistent with Git dates
+# Richard Brooksby, Ravenbrook Limited, 2023-03-23
+#
+# Copyright (c) 2023 Ravenbrook Limited.  See end of file for license.
+#
+# Only works for Ravenbrook copyrights.  TODO: Should perhaps look for
+# the last notice in a group of lines.
+
+this_year="$(date '+%Y')"
+
+{
+    find . -path './.git' -prune -o \
+	 -type f ! -name '*~' ! -name '#*#' -print |
+    # git ls-tree -r --name-only HEAD |
+    while read -r path; do
+	# Skip binary files
+	if ! file --brief -- "$path" | grep -q 'text'; then continue; fi
+	
+	# Debugging output
+	# echo 1>&2 "$path: checking..."
+
+	# Ask Git for the year of the last commit to the file
+	author_year=$(git log -1 --format="%ai" "$path" |
+			  sed --silent --regexp-extended \
+			      --expression='s/^([[:digit:]]{4})-.*$/\1/p')
+
+	# ``git log`` has empty output if the file is not in Git
+	if test -z "$author_year"; then
+	    author_year=0
+	fi
+
+	# Search for copyright notices and extract the end years
+	nl < "$path" |
+	    sed --silent --regexp-extended \
+		--expression='s/^[[:space:]]*([[:digit:]]+)[[:space:]].*Copyright[[:space:]]+(.*[[:space:]]+)?([[:digit:]]{4}[[:space:]]*-[[:space:]]*)?([[:digit:]]{4})[[:space:]]+Ravenbrook[[:space:]].*$/\1 \4/p' |
+	    while read -r line end_year; do
+		if test "$end_year" -eq "$author_year"; then
+		    continue
+		elif test "$author_year" -eq 0 -a "$end_year" -eq "$this_year"; then
+	            continue
+		fi
+		echo "$path $line $author_year $end_year"
+	    done
+    done
+} | {
+    code=0
+    while read -r path line author_year end_year; do
+	code=1
+	if test "$author_year" -ne 0; then
+	    year="Git author year $author_year"
+	else
+	    year="this year $this_year"
+	fi
+	echo 1>&2 "$path:$line: Copyright end year $end_year does not match $year"
+    done
+    exit $code
+}
+
+# A. REFERENCES
+#
+# [None]
+#
+#
+# B. DOCUMENT HISTORY
+#
+# 2023-03-23 RB Created in response to review brainstorm <https://github.com/Ravenbrook/mps/pull/179#issuecomment-1481486144>.
+#
+#
+# C. COPYRIGHT AND LICENSE
+#
+# Copyright (C) 2023 Ravenbrook Limited <https://www.ravenbrook.com/>.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are
+# met:
+#
+# 1. Redistributions of source code must retain the above copyright
+#    notice, this list of conditions and the following disclaimer.
+#
+# 2. Redistributions in binary form must reproduce the above copyright
+#    notice, this list of conditions and the following disclaimer in the
+#    documentation and/or other materials provided with the
+#    distribution.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
+# IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
+# TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
+# PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+# HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#
+#
+# $Id$