ex: am trying like this but it won't  workout. then how????
trigger acctrig on Account (before insert,after update) {
Account acc =new Account();
          if(acc.SLAExpirationDate__c != System.today())
             acc.addError('Error! enter only today date');
}
or
trigger acctrig on Account (before insert,after update) {
Account acc =new Account();
acc.today__c = date.today();
if(acc.SLAExpirationDate__c !=acc.today__c)
acc.addError('Error! enter only today date');
}

so pls share anather better sample code for this.

ex: am trying like this but it won't  workout. then how????
trigger acctrig on Account (before insert,after update) {
Account acc =new Account();
          if(acc.SLAExpirationDate__c != System.today())
             acc.addError('Error! enter only today date');
}
or
trigger acctrig on Account (before insert,after update) {
Account acc =new Account();
acc.today__c = date.today();
if(acc.SLAExpirationDate__c !=acc.today__c)
acc.addError('Error! enter only today date');
}

so pls share anather better sample code for this.

Take a number field on Account. Whnever a Contact is inserted for a particular account, number value in accoutn ahould be increamented by 1.