diff --git a/builtin/commit-graph.c b/builtin/commit-graph.c
index d1ab6625f6..03a8331e2b 100644
--- a/builtin/commit-graph.c
+++ b/builtin/commit-graph.c
@@ -143,7 +143,7 @@ static int graph_write(int argc, const char **argv)
 		OPT_INTEGER(0, "size-multiple", &split_opts.size_multiple,
 			N_("maximum ratio between two levels of a split commit-graph")),
 		OPT_EXPIRY_DATE(0, "expire-time", &split_opts.expire_time,
-			N_("maximum number of commits in a non-base split commit-graph")),
+			N_("only expire files older than a given date-time")),
 		OPT_END(),
 	};
 
diff --git a/commit-graph.c b/commit-graph.c
index f013a84e29..0d0d37787a 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -1707,7 +1707,7 @@ static void expire_commit_graphs(struct write_commit_graph_context *ctx)
 	timestamp_t expire_time = time(NULL);
 
 	if (ctx->split_opts && ctx->split_opts->expire_time)
-		expire_time -= ctx->split_opts->expire_time;
+		expire_time = ctx->split_opts->expire_time;
 	if (!ctx->split) {
 		char *chain_file_name = get_chain_filename(ctx->odb);
 		unlink(chain_file_name);
diff --git a/t/t5324-split-commit-graph.sh b/t/t5324-split-commit-graph.sh
index 53b2e6b455..b8b208fc3d 100755
--- a/t/t5324-split-commit-graph.sh
+++ b/t/t5324-split-commit-graph.sh
@@ -210,8 +210,14 @@ test_expect_success 'test merge stragety constants' '
 		git config core.commitGraph true &&
 		test_line_count = 2 $graphdir/commit-graph-chain &&
 		test_commit 15 &&
-		git commit-graph write --reachable --split --size-multiple=10 --expire-time=1980-01-01 &&
+		touch $graphdir/to-delete.graph $graphdir/to-keep.graph &&
+		test-tool chmtime =1546362000 $graphdir/to-delete.graph &&
+		test-tool chmtime =1546362001 $graphdir/to-keep.graph &&
+		git commit-graph write --reachable --split --size-multiple=10 \
+			--expire-time="2019-01-01 12:00 -05:00" &&
 		test_line_count = 1 $graphdir/commit-graph-chain &&
+		test_path_is_missing $graphdir/to-delete.graph &&
+		test_path_is_file $graphdir/to-keep.graph &&
 		ls $graphdir/graph-*.graph >graph-files &&
 		test_line_count = 3 graph-files
 	) &&