I tried using <?php echo $hm->Zb ('rs:def:rlog_edit_date_time');?> but its not returning me back with the last modified date time. Would you have any ideas why this would be?

Try this. Open web/codelib/asc/df.fl.(table-name).inc.php and
find the following array:

'rlog_edit_date_time'=>array(
XA_CLASS=>'cls_rlog_date_time',
XA_CAPTION=>RSTR_EDIT_DATE_TIME,
XA_FORMAT=>'Y-m-d H:i:s',
XA_LIST=>'(rlog)(edit_save)'
),

Add '(sr)' to XA_LIST. The reuslt should look like

'rlog_edit_date_time'=>array(
XA_CLASS=>'cls_rlog_date_time',
XA_CAPTION=>RSTR_EDIT_DATE_TIME,
XA_FORMAT=>'Y-m-d H:i:s',
XA_LIST=>'(rlog)(edit_save)(sr)'
),