/usr/share/postgresql/9.1/extension/moddatetime--unpackaged--1.0.sql is in postgresql-contrib-9.1 9.1.3-2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 | /* contrib/spi/moddatetime--unpackaged--1.0.sql */
-- complain if script is sourced in psql, rather than via CREATE EXTENSION
\echo Use "CREATE EXTENSION moddatetime" to load this file. \quit
ALTER EXTENSION moddatetime ADD function moddatetime();
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