diff --git a/t/helper/test-date.c b/t/helper/test-date.c
index 4727bea255..ac7c66c733 100644
--- a/t/helper/test-date.c
+++ b/t/helper/test-date.c
@@ -5,7 +5,8 @@ static const char *usage_msg = "\n"
 "  test-date show:<format> [time_t]...\n"
 "  test-date parse [date]...\n"
 "  test-date approxidate [date]...\n"
-"  test-date is64bit\n";
+"  test-date is64bit\n"
+"  test-date time_t-is64bit\n";
 
 static void show_relative_dates(const char **argv, struct timeval *now)
 {
@@ -96,6 +97,8 @@ int cmd_main(int argc, const char **argv)
 		parse_approxidate(argv+1, &now);
 	else if (!strcmp(*argv, "is64bit"))
 		return sizeof(unsigned long) == 8 ? 0 : 1;
+	else if (!strcmp(*argv, "time_t-is64bit"))
+		return sizeof(time_t) == 8 ? 0 : 1;
 	else
 		usage(usage_msg);
 	return 0;
diff --git a/t/t0006-date.sh b/t/t0006-date.sh
index 9539b425ff..42d4ea61ef 100755
--- a/t/t0006-date.sh
+++ b/t/t0006-date.sh
@@ -53,8 +53,8 @@ check_show unix-local "$TIME" '1466000000'
 
 # arbitrary time absurdly far in the future
 FUTURE="5758122296 -0400"
-check_show iso       "$FUTURE" "2152-06-19 18:24:56 -0400" TIME_IS_64BIT
-check_show iso-local "$FUTURE" "2152-06-19 22:24:56 +0000" TIME_IS_64BIT
+check_show iso       "$FUTURE" "2152-06-19 18:24:56 -0400" TIME_IS_64BIT,TIME_T_IS_64BIT
+check_show iso-local "$FUTURE" "2152-06-19 22:24:56 +0000" TIME_IS_64BIT,TIME_T_IS_64BIT
 
 check_parse() {
 	echo "$1 -> $2" >expect
diff --git a/t/t5000-tar-tree.sh b/t/t5000-tar-tree.sh
index 997aa9dea2..fe2d4f15a7 100755
--- a/t/t5000-tar-tree.sh
+++ b/t/t5000-tar-tree.sh
@@ -402,7 +402,7 @@ test_expect_success TIME_IS_64BIT 'generate tar with future mtime' '
 	git archive HEAD >future.tar
 '
 
-test_expect_success TAR_HUGE,TIME_IS_64BIT 'system tar can read our future mtime' '
+test_expect_success TAR_HUGE,TIME_IS_64BIT,TIME_T_IS_64BIT 'system tar can read our future mtime' '
 	echo 4147 >expect &&
 	tar_info future.tar | cut -d" " -f2 >actual &&
 	test_cmp expect actual
diff --git a/t/test-lib.sh b/t/test-lib.sh
index beee1d847f..8d25cb7c18 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -1166,3 +1166,4 @@ test_lazy_prereq LONG_IS_64BIT '
 '
 
 test_lazy_prereq TIME_IS_64BIT 'test-date is64bit'
+test_lazy_prereq TIME_T_IS_64BIT 'test-date time_t-is64bit'