lib/PublicInbox/Git.pm | 18 ++++++++++++++++++
 t/git.fast-import-data |  2 +-
 t/git.t                |  5 +++++

diff --git a/lib/PublicInbox/Git.pm b/lib/PublicInbox/Git.pm
index 8a96e10c1139be7d4d1b44e959e1ea9e3d676f38..236f70c165af6a275ab0898486cb7992700bae09 100644
--- a/lib/PublicInbox/Git.pm
+++ b/lib/PublicInbox/Git.pm
@@ -312,6 +312,24 @@ 	utf8::decode($$buf);
 	($$buf =~ /\r?\n\r?\n([^\r\n]+)\r?\n?/)[0]
 }
 
+# returns the modified time of a git repo, same as the "modified" field
+# of a grokmirror manifest
+sub modified ($) {
+	my ($self) = @_;
+	my $modified = 0;
+	my $fh = popen($self, qw(rev-parse --branches));
+	defined $fh or return $modified;
+	local $/ = "\n";
+	foreach my $oid (<$fh>) {
+		chomp $oid;
+		my $buf = cat_file($self, $oid) or next;
+		$$buf =~ /^committer .*?> (\d+) [\+\-]?\d+/sm or next;
+		my $cmt_time = $1;
+		$modified = $cmt_time if $cmt_time > $modified;
+	}
+	$modified || time;
+}
+
 1;
 __END__
 =pod
diff --git a/t/git.fast-import-data b/t/git.fast-import-data
index 4a105ee74a9bda233e40f51bce29c84f1f9de3b5..6d5159dda473c9b436d7b34f47e3b66396783faf 100644
--- a/t/git.fast-import-data
+++ b/t/git.fast-import-data
@@ -90,7 +90,7 @@
 commit refs/heads/master
 mark :13
 author AU Thor <e@example.com> 0 +0000
-committer AU Thor <e@example.com> 0 +0000
+committer AU Thor <e@example.com> 749520000 +0000
 data 18
 add git submodule
 from :11
diff --git a/t/git.t b/t/git.t
index d637e63ba8d49381941431e4966d3abd5d125823..5b612b8e42d9cab7792e84774579b55cbbd8615d 100644
--- a/t/git.t
+++ b/t/git.t
@@ -23,6 +23,7 @@ }
 
 {
 	my $gcf = PublicInbox::Git->new($dir);
+	is($gcf->modified, 749520000, 'modified time detected from commit');
 	my $f = 'HEAD:foo.txt';
 	my @x = $gcf->check($f);
 	is(scalar @x, 3, 'returned 3 element array for existing file');
@@ -146,6 +147,10 @@
 	ok($gcf->cleanup(time - 30), 'cleanup did not expire');
 	ok(!$gcf->cleanup(time + 30), 'cleanup can expire');
 	ok(!$gcf->cleanup, 'cleanup idempotent');
+
+	my $t = $gcf->modified;
+	ok($t <= time, 'repo not modified in the future');
+	isnt($t, 0, 'repo not modified in 1970')
 }
 
 use_ok 'PublicInbox::Git', qw(git_unquote git_quote);