Why was time important in pairing the UCS with the CS?

The relationship between UCS (unconditional stimulus) and the CS (conditioned stimulus) is extremely time-sensitive because for an association to form between the two, a certain amount of time needs to pass between the onset of the UCS and the onset of the CS. There are two styles of conditioning that will elicit a CR (conditioned response) when the time period between UCS and CS are carefully controlled, which are:

Delayed conditioning: Occurs when CS onset precedes UCS but continues until UCS is presented. For example, when a bell rings and does not stop ringing until water is presented. Trace conditioning: Occurs when CS onset precedes UCS but stops just before the UCS is presented. Using the bell as the example, the bell will ring and will stop ringing 2 seconds before the water is presented.