reading an EEG dataset using EEGLAB When I'm reading the documentation of importing events in EEGLAB from a file here I have noticed that when we used a stimulus , we must know which time  we will apply it , since in the dataset file , you have to identify  of what times stimulus happen .  I don't know if my understanding is right or not , since i didn't  deal with EEG data practically (but in 2 weeks I will), but I was  reading a theoretical topics and papers on that.
 thanks in advance.
If I understand your question correctly, yes, since ERP involves averaging over similar events, very accurate time-locking to stimulus onset is critical.  The idea of an ERP is that activity relevant to the stimulus will happen at a similar timepoint after similar stimuli--if you don't have an accurate time stamp (usually to the level of single milliseconds if not better), then the activity will be "smeared" across the time course.  For a good overview of ERPs, see the article Event-related Potentials: An Overview