lastmod is on by default.

doc/man/man5/slapd.conf.5

-.TH SLAPD.CONF 5 "20 January 1999" "OpenLDAP LDVERSION"
+.TH SLAPD.CONF 5 "5 August 1999" "OpenLDAP LDVERSION"
 .SH NAME
 slapd.conf \- configuration file for slapd, the stand-alone LDAP daemon
 .SH SYNOPSIS
@@ -263,7 +263,7 @@ Controls whether
 .B slapd
 will automatically maintain the 
 modifiersName, modifyTimestamp, creatorsName, and 
-createTimestamp attributes for entries.  By default, lastmod is off.
+createTimestamp attributes for entries.  By default, lastmod is on.
 .TP
 .B readonly on | off
 This option puts the database into "read-only" mode.  Any attempts to