What field is Views using to determine the pager 'offset'? When I use Views > pager > 'items to display' and set the offset value ... what field is Views using to determine the pager 'offset'? Is it 'post date'? 

How can I change the offset to use a different date field that I've created?
It offsets by row. 

If offset = 5, it will skip the first 5 rows of your view result.

By default, the View SORT is using Post Date. 

Remove Post Date and add the desired field to the SORT area. (when adding, look for your custom date field that you created.)