You'll need to modify the theme.
1. Open base.rc
2. Find the variable DateLabelText and change it to
"[uptime('d')]d [uptime('h')]:[uptime('nn')]:[uptime('ss')]"
3. Change the variable DateLabelOnLeftClick to
!DateLabelSetText "[date('ddd')]. [date('mmm')]. [date('d')]"
4. Change the variable DateLabelOnRightClick to
!DateLabelSetText "[uptime('d')]d [uptime('h')]:[uptime('nn')]:[uptime('ss')]"
5. Now save and recycle LS :)