To add to automarc's reply;
If you are set at 12 degrees BTDC (12 degrees advanced from TDC), and you retard the timing (towards TDC) 4 degrees, you are now set at 8 degrees BTDC (8 degrees advanced from TDC.
12 degrees (BTDC or advanced)
- 4 degrees (retarded)
________________________
8 degrees BTDC (or 8 degrees advanced)
