trigger AssetTrigger on Asset (after delete, after insert, after update) {
	/**	We need to use the after contexts in this trigger, since if some sort of exception were to
	cause dml to fail, then we could possibly recalculate the number assuming that the dml succeeded,
	it fails, and we're left with an incorrectly calculated number.
	**/
	if (trigger.isDelete || trigger.isInsert) {
		/**	We need to build a list of the record ids in trigger.new that match our criteria
		so we can get all of the Accounts that we need to recalculate in one SOQL query in the
		event that we have a large batch from something like a batch data load.
		**/
		Set<Id> accountIds = new Set<Id>();
		for (Asset asset : trigger.new) {
			if (System.today() - asset.SalesDate__c <= 365) { //This is probably not what you need, but you get the point.
				accountIds.add(asset.AccountId);
			}
		}
		/** Now that we have all of the account ids that we care about, we'll get all of the 
		Accounts and all of their assets so that we can just use .size() on the Assets collection.
		**/
		List<Account> accountsThatNeedRecalculation = [Select Id, (Select Id From Asset Where TODAY() - SalesDate__c <= 365) From Account Where Id IN: accountIds];
		for (Account account : accountsThatNeedRecalculation) {
			account.CalculatedField__c = account.Assets.size();
		}

		try {
			update accountsThatNeedRecalculation;
		}
		catch (SomeException ex) {
			//Do something to handle this.
		}
	}
	else if (trigger.isUpdate) {
		/** Nearly the same logic as insert or delete contexts, but you need to use trigger.oldMap
		to determine if the record has been modified to meet the need to recalculate, not just meets the need
		since it's possible that some other field was edited. In that case, we don't care.
		**/
		Set<Id> accountIds = new Set<Id>();
		for (Asset asset : trigger.new) {
			if (System.today() - asset.SalesDate__c <= 365 && asset.SalesDate__c != trigger.oldMap.get(asset.Id).SalesDate__c) {//The second part of this is the only difference.
				accountIds.add(asset.AccountId);
			}
		}
		//Same as above.
	}
}