When using the 'Sort' Group of user attributes ($Sort, $SortAlso) to sort on any Date-type attribute, any default not-yet-set "never" dates are listed last. This the same for both note and agent based container sorting.
If the sort is reversed (suing ($SortBackward and $SortBackwardAlso), the "never" dates list first.
This indicates Tinderbox is using a lexical sort—sorting the dates as if literal text strings as opposed to using their implied date numbering. The linked page explains the logic of lexical sort.
A separate article covers sorting Date-type data using Action code.
Legacy note: In older versions of Tinderbox, the "never" dates always listed first regardless of the sort/reverse order. That reflected user requests to make non-set dates easier to find. It appears this older behaviour no longer occurs. However, if encountered, that it the reason for the difference form the above.