I don't clearly understand what you do want. Looks like rcm has three choice: Today, short date in same week or long date time format for weeks ago.
A coarse resolution could apply on program/include/rcmail.php
1192 #if ($pretty_date && $timestamp > $today_limit && $timestamp <= $now) {
1193 # $format = $this->config->get('date_today', $this->config->get('time_format', 'H:i' ));
1194 # $today = true;
1195 #}
1196 #else if ($pretty_date && $timestamp > $week_limit && $timestamp <= $now) {
1197 # $format = $this->config->get('date_short', 'D H:i');
1198 #}
1199 #else {
1200 # $format = $this->config->get('date_long', 'Y-m-d H:i');
1201 #}
1202 $format = $this->config->get('date_long', 'Y-m-d H:i');
And a screen shot looks like the attachment.
Hope you know how to modify the code.
Good luck!