Based on functionality in V1.30.3 and above
The standard AD tools don't expose the time when a member is added or removed from a group, and the normal method is to use the security event log to retrieve these details, however, this makes the assumptions that auditing was enabled when the change was made and the security event log hasn't wrapped and the details are still available, which is not always the case.
There is another way to get this information that doesn't rely on auditing being enabled or the size of the security event log to capture the details. The AD does maintain when changes happened in the replication data for group objects, and this data contains the exact details of the time when these membership changes occurred. The AD uses this information enable changes to be replicated to other domain controllers in the domain or forest. The replication data is not easily accessible with the standard AD tools, however NetTools has a simple feature to allows you to display all the membership changes for a group, including the time they happened. The time a member was added or removed shown in corresponding column.
The option is available on the Members tab in the AD Properties dialog, at the bottom of the tab is the Changes button, when this is clicked a separate window is displayed with all the change details.