Custom event helper to do the logic

CustomHelper.cls

public class CustomEventHelper {

    public Boolean timeAv(DateTime s, DateTime e, Id accId, Id cid) {
        Boolean allowed = false;
        try {
            Long c = [SELECT count() FROM CustomEvent__c WHERE Account__c=:accId 
            AND 
            (
                (StartDate__c <=:s AND EndDate__c >=:s) OR 
                (StartDate__c <=:e AND EndDate__c >=:e) OR 
                (StartDate__c>=:s AND EndDate__c<=:e)
            )
            AND Id<>:cid];
            
            allowed = c == 0;
        } catch (Exception ex){
            system.debug('Error ' + ex.getMessage());
        }
        return allowed;
    }
}