If you simply wanted the DUE date to be a certain number days after the START date, then you could do something like this:
Code:
set DAYS_FROM_START to 5
--tell application "Finder" to set theFile to (first item of (selection as list)) as alias
tell application "Finder" to set file_name to (POSIX path of theFile)
set dteBase to current date
set theNote to "Scanned " & (dteBase as string) & "
"
set projectname to "Factures à Encoder"
set ContextName to "En-Ligne"
set theTask to "Encoder: \"" & file_name & "\""
tell application "OmniFocus"
set task_title to theTask
tell default document
-- GET A REFERENCE TO AN EXISTING PROJECT (OR NEW) PROJECT
set lstProj to flattened projects where name = projectname
if lstProj ≠ {} then
set oProj to item 1 of lstProj
else
set oProj to (make new project with properties {name:projectname})
end if
-- GET A REFERENCE TO AN EXISTING (OR NEW) CONTEXT
set lstContexts to flattened contexts where name = ContextName
if lstContexts ≠ {} then
set oContext to item 1 of lstContexts
else
set oContext to (make new context with properties {name:ContextName})
end if
-- MAKE A NEW TASK UNDER THE SPECIFIED PROJECT,
-- ATTACHING IT TO THE SPECIFIED CONTEXT
-- AND STAMPING IT WITH A START DATE
set dteStart to dteBase - (time of dteBase) -- zero the time to midnight
set dteStart to dteStart + (10 * days) -- and add three days
set dteDue to dteStart + (DAYS_FROM_START * days) -- Due N days after start
tell oProj
tell (make new task with properties {name:task_title, context:oContext, start date:dteStart, due date:dteDue, flagged:true})
set its note to theNote
tell its note to make new file attachment with properties {file name:file_name, embedded:true}
end tell
end tell
end tell
end tell