_in_x_day
is key since 6.1.0
and have value In {0} Day{1}
look here:
INSERT INTO `LocalizationKeys` VALUES(2791, 1, '_x_minute_ago');
INSERT INTO `LocalizationKeys` VALUES(2792, 1, '_x_hour_ago');
INSERT INTO `LocalizationKeys` VALUES(2793, 1, '_x_day_ago');
INSERT INTO `LocalizationKeys` VALUES(2794, 1, '_in_x_minute');
INSERT INTO `LocalizationKeys` VALUES(2795, 1, '_in_x_hour');
INSERT INTO `LocalizationKeys` VALUES(2796, 1, '_in_x_day');

INSERT INTO `LocalizationStrings` VALUES(2791, see more 1, '{0} Minute{1} Ago');
INSERT INTO `LocalizationStrings` VALUES(2792, 1, '{0} Hour{1} Ago');
INSERT INTO `LocalizationStrings` VALUES(2793, 1, '{0} Day{1} Ago');
INSERT INTO `LocalizationStrings` VALUES(2794, 1, 'In {0} Minute{1}');
INSERT INTO `LocalizationStrings` VALUES(2795, 1, 'In {0} Hour{1}');
INSERT INTO `LocalizationStrings` VALUES(2796, 1, 'In {0} Day{1}');

so it was include even in 6.1