diff --git a/dev-utils/git-date.py b/dev-utils/git-date.py
new file mode 100755
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--- /dev/null
+++ b/dev-utils/git-date.py
@@ -0,0 +1,337 @@
+#! /usr/bin/env python3
+# -*- coding: utf-8 -*-
+
+# git-date.py --- Find Git commits around some date in one or more repositories.
+# Copyright (c) 2021, Florent Rougon
+# All rights reserved.
+#
+# 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 OWNER 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.
+#
+# The views and conclusions contained in the software and documentation are
+# those of the authors and should not be interpreted as representing official
+# policies, either expressed or implied, of the FlightGear project.
+
+# The idea and some Git-fu of this script are from Edward d'Auvergne:
+# <https://sourceforge.net/p/flightgear/mailman/message/37004175/>.
+
+import argparse
+import locale
+import os
+import re
+import subprocess
+import sys
+from collections import namedtuple, OrderedDict
+
+PROGNAME = os.path.basename(sys.argv[0])
+PROGVERSION = "0.2"
+COPYRIGHT = "Copyright (c) 2021, Florent Rougon"
+LICENSE_SUMMARY = """\
+This program is free software. It comes without any warranty, to
+the extent permitted by applicable law. See the top of {progname}
+for more details on the licensing conditions.""".format(progname=PROGNAME)
+
+# Very simple Repository class
+Repository = namedtuple('Repository', ['label', 'path'])
+
+
+class CommitFinder:
+
+    def __init__(self, branch, date):
+        self.branch = branch
+        self.date = date
+
+    def findCommit(self, repo_path):
+        """Return a commit ID that belongs to 'repo_path'."""
+        args = ["git", "rev-list", "--max-count=1",
+                "--before={}".format(self.date), self.branch]
+        p = subprocess.run(args, cwd=repo_path, capture_output=True,
+                           check=True, encoding="utf-8")
+        return p.stdout.strip()
+
+    def action(self, repositories):
+        """Act on one or more repositories.
+
+        'repositories' should be an OrderedDict whose keys are
+        repository labels and values Repository objects.
+
+        """
+        for label, repo in repositories.items():
+            commitId = self.findCommit(repo.path)
+            if params.let_me_breathe: self.print("-" * 78)
+            if not params.checkout: # the output would be redundant
+                if params.only_label:
+                    # Useful with --show-commits
+                    #             --show-commits-option='--no-patch'
+                    #             --show-commits-option='--format=oneline'
+                    self.print("{}: ".format(label), end='')
+                else:
+                    self.print("{}: {}".format(label, commitId))
+                if params.let_me_breathe: self.print()
+
+            if params.show_commits:
+                args = ["git", "-c", "pager.show=false", "show"] + \
+                       params.show_commits_options + [commitId]
+                subprocess.run(args, cwd=repo.path, check=True)
+
+            if params.checkout:
+                args = ["git", "checkout", commitId]
+                self.print("{}: checking out commit {}...".format(label,
+                                                                  commitId))
+                subprocess.run(args, cwd=repo.path, check=True)
+
+            if params.let_me_breathe: self.print()
+
+    def print(self, *args, **kwargs):
+        """Wrapper for print() that defaults to flushing the output stream.
+
+        This is particularly useful when stdout is fully buffered (e.g.,
+        when piping the output of the script through a pager). Without
+        this 'flush=True' setting, output from Git commands would bypass
+        the high-level buffering layer in sys.stdout and could come out
+        before the output of some *later* non-flushed print()
+        statements.
+        """
+        print(*args, flush=True, **kwargs)
+
+
+def parseConfigFile(cfgFile, configFileOptSpecified, recognizedParams):
+    namespace = argparse.Namespace()
+    l = {}
+    if configFileOptSpecified or os.path.exists(cfgFile):
+        # Read the configuration file (i.e., execute it)
+        with open(cfgFile, "r") as f:
+            exec(f.read(), {"OrderedDict": OrderedDict}, l)
+
+    for p in recognizedParams:
+        if p in l:
+            setattr(namespace, p, l[p])
+
+    return namespace
+
+
+def processCommandLineAndConfigFile():
+    defaultCfgFile = os.path.join(os.getenv('HOME'),
+                                  ".config", PROGNAME, "config.py")
+    parser = argparse.ArgumentParser(
+usage="""\
+%(prog)s [OPTION ...] DATE [REPOSITORY...]
+Find Git commits before DATE in one or more repositories.""",
+        description="""\
+Print information about, and possibly check out the most recent commit
+before DATE in each of the specified repositories. By default, commits
+are searched for in the 'next' branch, however this can be changed using
+the --branch option or the 'branch' variable in the configuration file.
+DATE can be in any date format accepted by Git (see the examples below).
+
+If option --repo-args-are-just-paths has been given, each REPOSITORY
+argument is literally treated as a path to a repository. Otherwise, each
+REPOSITORY argument that has the form LABEL=PATH defines a repository
+rooted at PATH with associated LABEL (using this special syntax is not
+mandatory, but allows {progname} to refer to your repositories using the
+provided labels, which is more user-friendly in general).
+
+Examples (the backslashes just introduce continuation lines):
+
+# One output line per repository (terse)
+{progname} "2021-02-28 23:12:00" SG=/path/to/SG \\
+            FG=/path/to/FG FGData=/path/to/FGData
+
+# Ditto without providing the repository labels
+{progname} "2021-02-28 23:12:00" /path/to/SG \\
+            /path/to/FG /path/to/FGData
+
+# Run 'git show' with the specified options for each commit found.
+{progname} --let-me-breathe --show-commits \\
+            --show-commits-option='--no-patch' \\
+            --show-commits-option='--format=medium' \\
+            '2021-02-28 23:12:00' SG=/path/to/SG \\
+            FG=/path/to/FG FGData=/path/to/FGData
+
+# Run 'git checkout' for each commit found.
+{progname} --checkout --let-me-breathe "2021-01-01" SG=/path/to/SG \\
+            FG=/path/to/FG FGData=/path/to/FGData
+
+# For each repository, print the label, commit ID and one-line description.
+{progname} --only-label --show-commits \\
+            --show-commits-option='--no-patch' \\
+            --show-commits-option='--format=oneline' \\
+            "2021-02-28" SG=/path/to/SG \\
+            FG=/path/to/FG FGData=/path/to/FGData
+
+Note: --show-commits et al. may be used in conjunction with --checkout
+      if so desired.
+
+If $HOME/.config/{progname}/config.py exists or if the --config-file option
+has been given, a configuration file is read. This file is executed by
+the Python interpreter and must therefore adhere to Python 3 syntax.
+Here is a sample configuration file:
+
+------------------------------------------------------------------------------
+branch = 'release/2020.3'
+# checkout = True
+# show_commits = True
+# show_commits_options = ['--no-patch', '--format=medium']
+# let_me_breathe = True
+# only_label = True
+# repo_args_are_just_paths = True
+
+# collections.OrderedDict is available for use here:
+repositories = OrderedDict(
+    SimGear    = "/path/to/simgear",
+    FlightGear = "/path/to/flightgear",
+    FGData     = "/path/to/fgdata")
+
+# Same list of repositories but without user-defined labels:
+# repositories = [
+#     "/path/to/simgear",
+#     "/path/to/flightgear",
+#     "/path/to/fgdata"]
+------------------------------------------------------------------------------
+
+Command-line options take precedence over their counterparts found in
+the configuration file. On the other hand, REPOSITORY arguments *extend*
+the list of repositories that may be defined in the configuration file
+using the 'repositories' variable.""".format(progname=PROGNAME),
+        formatter_class=argparse.RawDescriptionHelpFormatter,
+        # I want --help but not -h (it might be useful for something else)
+        add_help=False)
+
+    parser.add_argument('--repo-args-are-just-paths',
+                        action='store_true', help="""\
+      don't try to recognize and special-case LABEL=PATH syntax for
+      repository arguments; treat them literally as paths and simply assign
+      labels 'Repo 1', 'Repo 2', etc., to the specified repositories""")
+    parser.add_argument('-b', '--branch', default="next", help="""\
+      search history of BRANCH (default: %(default)s)""")
+    parser.add_argument('-c', '--checkout', action='store_true', help="""\
+      run 'git checkout' for the commit that was found in each repository""")
+    # This option is actually handled by configFileOptParser because we want to
+    # treat it before all other options.
+    parser.add_argument('--config-file', metavar="FILE", default=defaultCfgFile,
+                        help="""\
+      load configuration from FILE  (default: %(default)s""")
+    parser.add_argument('-s', '--show-commits', action='store_true', help="""\
+      run 'git show' for the commit that was found in each repository""")
+    parser.add_argument('-S', '--show-commits-option', action='append',
+                        dest='show_commits_options', help="""\
+      option passed to 'git show' when --show-commits is used (may be
+      specified multiple times, as in: --show-commits-option='--no-patch'
+      --show-commits-option='--format=medium')""")
+    parser.add_argument('--let-me-breathe', action='store_true', help="""\
+      add blank lines and other separators to make the output hopefully more
+      readable when Git prints a lot of things""")
+    parser.add_argument('--only-label', action='store_true', help="""\
+      don't print the commit ID after the repository label (this is useful
+      when the Git output coming next already contains the commit ID)""")
+    parser.add_argument('date', metavar="DATE", help="""\
+      find commits before this date""")
+    parser.add_argument('cmdRepos', metavar="REPOSITORY", nargs='*',
+                        help="""\
+      path to a repository to act on (as many arguments of this type as desired
+      can be given)""")
+    parser.add_argument('--help', action="help",
+                        help="display this message and exit")
+    parser.add_argument('--version', action='version',
+                        version="{name} version {version}\n{copyright}\n\n"
+                                "{license}".format(
+                                    name=PROGNAME, version=PROGVERSION,
+                                    copyright=COPYRIGHT,
+                                    license=LICENSE_SUMMARY))
+
+    # Find which config file to read and note whether the --config-file option
+    # was given.
+    configFileOptParser = argparse.ArgumentParser(add_help=False)
+    configFileOptParser.add_argument('--config-file')
+    ns, remaining = configFileOptParser.parse_known_args()
+    if ns.config_file is not None:
+        configFileOptSpecified = True
+    else:
+        configFileOptSpecified = False
+        ns.config_file = defaultCfgFile
+
+    recognizedParams = ("repo_args_are_just_paths", "branch", "checkout",
+                        "show_commits", "show_commits_options",
+                        "let_me_breathe", "only_label", "repositories")
+    # Read the config file into 'params' (an argparse.Namespace object)
+    params = parseConfigFile(ns.config_file, configFileOptSpecified,
+                             recognizedParams)
+
+    # Process the rest of the command-line
+    parser.parse_args(namespace=params)
+
+    if "repositories" not in params:
+        sys.exit(f"{PROGNAME}: no repository was specified, neither in the "
+                 "configuration file\nnor on the command line; exiting.")
+
+    # Prepare the final list of repositories based on the config file and the
+    # command line arguments.
+    params.repositories = initListOfRepositories(
+        params.repositories, params.cmdRepos, params.repo_args_are_just_paths)
+    return params
+
+
+# Returns an OrderedDict whose keys are repository labels and values Repository
+# objects.
+def initListOfRepositories(reposFromCfgFile, reposFromCmdLineArgs,
+                           repoArgsAreJustPaths):
+    res = OrderedDict()
+    reposLeftToAdd = []
+
+    if isinstance(reposFromCfgFile, OrderedDict):
+        for label, path in reposFromCfgFile.items():
+            res[label] = Repository(label, path)
+    elif isinstance(reposFromCfgFile, list):
+        reposLeftToAdd.extend(reposFromCfgFile)
+    else:
+        sys.exit(f"{PROGNAME}: in the configuration file, 'repositories' must "
+                 "be either an OrderedDict or a list.")
+
+    repoNum = len(res)
+    for elt in reposLeftToAdd + reposFromCmdLineArgs:
+        repoNum += 1
+        mo = re.match(r"^(?P<label>\w+)=(?P<path>.*)", elt)
+        if mo is None or repoArgsAreJustPaths:
+            label = "Repo {}".format(repoNum)
+            path = elt
+        else:
+            label, path = mo.group("label", "path")
+        res[label] = Repository(label, path)
+
+    return res
+
+
+def main():
+    global params
+
+    locale.setlocale(locale.LC_ALL, '')
+    # Require Python 3.6 or later because we rely on the retained order for
+    # keyword arguments passed to the OrderedDict constructor.
+    if sys.hexversion < 0x030600F0:
+        sys.exit(f"{PROGNAME}: exiting because Python >= 3.6 is required.")
+
+    params = processCommandLineAndConfigFile()
+    commitFinder = CommitFinder(params.branch, params.date)
+    commitFinder.action(params.repositories)
+    sys.exit(0)
+
+
+if __name__ == "__main__": main()