Excessive valve clearances will cause the duration of valve opening to:

Excessive valve clearances primarily lead to a reduction in the duration of valve opening for both intake and exhaust valves. When valve clearance is too large, the valves do not fully close for the duration necessary during the engine cycle. This results in a shorter effective opening time for the valves because they spend a portion of their cycle in motion adjusting to the clearance rather than performing their specific function of allowing air-fuel mixture in and exhaust gases out efficiently.

Furthermore, the engine timing can be affected by this excess clearance, which can lead to less efficient combustion and a loss of power. With the clearance being excessive, the critical timing at which the valve opens and closes is altered, essentially "cutting short" the time during which the valve is fully open, hence decreasing the effective duration of the opening.

Understanding valve timing is crucial in engine performance; therefore, it's important to manage clearances diligently, maintaining them within manufacturer's specifications to ensure optimal function and efficiency of the engine. This is why the answer indicates a decrease in valve opening duration for both types of valves.