I sorted it out and this is what worked for me:
Value Field:
posttime ASC;;Ascending
posttime DESC;;Descending
Filter Clause:
Must
Filter is conditional:
Unchecked
I had an issue where the sorted result was not persisting upon page change. I realized through trial and error that Filter auto postback was the culprit. I had this checked, this needs to be unchecked if you're loading more than one page.