If I would need to substract different date attributes I would use vql functions like getday(date), gethour(date), getminute(date), getsecond(date)... as they return long values you could substract them without any problem.
I used a vql query to substract the days and the hours from two dates because I needed to know how many days and hours were between those dates:
*SELECT mytable."ID", mytable."ENDDATE" - mytable."INITDATE" as "daysdifference", (gethour( mytable."ENDDATE") - gethour( mytable."INITDATE")) as "hoursdifference" FROM mytable;*
I think that you could do something similar if you need to get the time difference. You could take a look into the "*Advanced VQL guide*". There is more information at the "*Date Processing Functions*" section.
Hope this helps!