Apex Errors not functioning

if(res.Start__c < system.now()){
 ApexPages.addmessage(new ApexPages.message(ApexPages.severity.WARNING,' Start Time should always be greater than current time.'));                    
            }
 if(res.end__c < system.now()){
 ApexPages.addmessage(new ApexPages.message(ApexPages.severity.WARNING,' End Time should always be greater than current time.'));   
            }

I also used  <apex:pageMessages/> in VF page.