Issues with "diff" and date/time

Hello,

I'm trying to display a custom chart based on the following SWQL query:

select MAX(inbps) - MIN(inbps) AS diff

FROM Orion.NPM.Interfaces

WHERE interfaceid IN (510,27429)

I am, however, having issues displaying this in a chart view.  It asks me to specify a date/time, but this doesn't not show up as an option when trying to add a new column.

In the end, essentially what I need to do is display the difference in bandwidth utilized between 2 different interfaces...

TIA for the help!

Scott