Timing Advance is BTDC. Before Top Dead Center. Spark happens before the piston hits top of the cylinder. (Nissan actually determines in some slightly different method, but irrelevant here). 15 degrees BTDC is the standard, but Nissan range is spec'd 10 to 20 as normal. This is the timing at Idle. As you accelerate/decelerate, the timing varies. You will see readings then, of 40 degrees or more, as Lubecakes said. Be certain you are reading at idle only. Timing Advance typically takes the initial timing and adjust the 15, to 17 BTDC for a bit quicker throttle response. It does not affect the rest of the timing curve. A CAI will never change your timing. Sometimes, the use of a - or + is confusing in conversations on timing and we get it all backwards. Negative Timing is ATDC. Timing is BTDC generally speaking. It takes time (excuse the pun) for the spark to fully ignite the fuel as the piston is flying up and down, that is why it sparks before the piston is up top.