Hi Lisa, there really isn't a great way to do it without a custom object. Initially I had implemented a solution that queried the highest value that currently existed on the object in question, and then incremented that by one as the starting point. The problem you run into with that solution is if  2 people are creating accounts or whatever at the exact same time (which happens more than you think), each apex transaction is not aware of the other one and you end up with duplicate numbers. Having an auto number field solves that, but if you only want to have it auto number sometimes you need a separate object to track that incrementing. Here's an apex solution that still uses a custom object:
 

trigger Account_Triggers on Account (before insert) {
    if (Trigger.isBefore and Trigger.isInsert){
        List<Account> accountsToAddNumTo = new List<Account>();
        List<Customer_Account_Number__c> accountNums = new List<Customer_Account_Number__c>();

        String timestamp = date.today().format('**yyyy-MM-dd HH:mm:ss**');

        for (Account a : Trigger.new){
            if (a.RecordType.Name = 'Customer'){
                accountsToAddNumTo.add(a);
                Customer_Account_Number__c num = new Customer_Account_Number__c(Transaction_Timestamp__c = timestamp);
                accountNums.add(num);
            }
        }
        if (accountNums.size() > 0){
            insert accountNums;
            List<Id> idList = new List<Id>();
            for (Customer_Account_Number__c an : accountNums){
                idList.add(an.Id);
            } 
            accountNums = [SELECT Id, Name FROM Customer_Account_Number__c WHERE Id IN :idList];

            Integer counter = 0;
            for (Account a: accountsToAddNumTo){
                a.Customer_Account_Number__c = accountNums[counter].Name;
                counter += 1;
            }
        }
    }
}