apex trigger before update

As per your requirement I have written the trigger below:

trigger checkDates on Agreement_ABC__c(before update) {
	Set<Id> setOfAgreements = new Set<Id>();
	for(Agreement_ABC__c agreement : trigger.new){
		//This conditon will check if start date or end date is updated on Agreement
		if(trigger.oldMap.get(agreement.id).agreement_start_date__c != agreement.agreement_start_date__c || trigger.oldMap.get(agreement.id).agreement_end_date__c != agreement.agreement_end_date__c) {
			//This set will store ids of only those agreements where start date or end date is updated
			setOfAgreements.add(agreement.id);
		}
	}
	//If there is any agreement whose start date or end date is updated then we will execute this block
	if(setOfAgreements.size() > 0) {
		//This will contains the ids of all the agreements which consists of an error.
		Set<Id> setOfAgreementWithError = new Set<Id>();
		Date agreementStartDate;
		Date agreementEndDate;
		Integer counterForStartDate;
		Integer counterForEndDate;
		//This will query all the agreements with their respective Jobs
		//Replcae Jobs__r with child relationship name
		for(Agreement_ABC__c agreement : [Select Id, (Select job_start_date__c, job_end_date__c from Jobs__r ) from Agreement_ABC__c where Id IN :trigger.new]) {
			//Fetch updated start date and  end date from trigger.new map
			agreementStartDate = trigger.newMap.get(agreement.id).agreement_start_date__c;
			agreementEndDate = trigger.newMap.get(agreement.id).agreement_end_date__c;
			counterForStartDate = 0;
			counterForEndDate = 0;
			for(Job__c job : agreement.Jobs__r) {
				if(agreementStartDate <= job.job_start_date__c) {
					counterForStartDate++;
				}
				if(agreementEndDate >= job.job_end_date__c) {
					counterForEndDate++;
				}
			}
			if(agreement.Jobs__r.size() > 0 && (counterForStartDate != agreement.Jobs__r.size() || counterForEndDate != agreement.Jobs__r.size()) ) {
				setOfAgreementWithError.add(agreement.id);
			}
		}
		for(Agreement_ABC__c agreement : trigger.new) {
			if(setOfAgreementWithError.containsKey(agreement.id)) {
				agreement.addError('Enter your Error Message here');
			}
		}
	}
}
//Please take care of the API names for all the field and objects.

Please let me know if you face any issue with this and if you need any further help or information on this.