In order to provide appropriate feedback, the user should be able to see that the item has been added to the list, so whether you add it to the top or the bottom - the list should scroll to show the newly added item.
Now whether it goes to the top or the bottom largely depends on whether the current items in the list are sorted [and the example of an associated timestamp implies chronological ordering].
Most lists of any size should be sorted so that the user can scan the list more easily using an attribute that is meaningful to them - eg by alphabitical order, date, time, etc, so the question is mute if there is already an ordering to the list - you simply insert according to the sorting rules. [then scroll to the newly added item - even if this involves multiple pages - the user sees the new item placed immediately into context]
If there really is no order and the list is completely random, then first or last simply has no significance, therefore take other clues (eg consistency: what do other lists in your application do; familiarity: what do competitors do; history: how were all the existing items added to the list; etc etc). There has to be some reason for the items in the list to be the order they are?
If it isn't already, then I'd suggest that the list is sorted in some way otherwise the list might be differently arranged every time you come back to it and that's a big no-no.