Track of account records into alternate contact (custom object) records

Question

We need to track any change in billing Address ( address,city,state,zip) of Account in a new Object : ÄLTERNATE CONTACT (Address, City,State, Zip, UpdatedDateTime, Account) . This object should only store unique address information. If the same address already exists in Alternate contact, just update “UpdatedDateTime” field without creating a new record.Only required on after update trigger

Answer 1

I'd put something like this in the trigger :

List<Id> ids = new List<Id>(); //ids of accounts with watched fields modified
for (Integer i=0; i<trigger.new.size(); i++) //foreach account
{
  if ( //if one of your fields is modified
       (trigger.old[i].address != trigger.new[i].address) || 
       (trigger.old[i].Zip != trigger.new[i].Zip) // || etc. with all field u want
     )
     {
        ids.add(trigger.new[i].Id); //keep track of its id
     }
}
if (ids.size()>0) //if u got accounts modified
{
  AlternateContact__c[] altCts = [Select Id, UpdatedDateTime__c from AlternateContact__c where AccountId__c in :ids]; //select alt contacts linked to these accounts
  for (AlternateContact__c ac : altCts) { //for each of them 
    ac.UpdatedDateTime__c = DateTime.now(); //update the date
  }
  update altCts; //save your changes
}

Please have a look at https://salesforce.stackexchange.com/help before you post your next question. This one lacked details.

Edit after your comment : If I understood you need to

1. Query all fields on your AlternateContact__c object related to account
2. Check if all the fields (except id of course) match your new address
3. if it does update date
4. if it doesn't create new AlternateContact__c

I can see nothing hard in it, you just have to adapt the code I wrote up there.