shadow/man/login.defs.d/ENV_TZ.xml

<varlistentry condition="no_pam">
  <term><option>ENV_TZ</option> (string)</term>
  <listitem>
    <para>
      If set, it will be used to define the TZ environment variable when
      a user login. The value can be the name of a timezone preceded by
      <replaceable>TZ=</replaceable> (for example
      <replaceable>TZ=CST6CDT</replaceable>), or the full path to the file
      containing the timezone specification (for example
      <filename>/etc/tzname</filename>).
    </para>
    <!-- TODO: it can in fact be used to set any other variable-->
    <para>
      If a full path is specified but the file does not exist or cannot be
      read, the default is to use <replaceable>TZ=CST6CDT</replaceable>.
    </para>
  </listitem>
</varlistentry>