Not at the moment, this request has been logged with our Development team for investigation.
However, sorting Purchase Order numbers in descending order will sort the list very closely to how sorting by date would.
Please see below steps:
From 'Dashboard', go to [Documents] [Purchase Order]. Click on 'Sent' tab.
Click on the drop-down arrow next to 'Number', click on 'Sort Descending' and Purchase Plus will sort the Purchase Orders from most recently sent to oldest sent. This setting will save itself automatically.