[whpalmer4]

You're asking Applescript to know how OmniFocus interprets date strings, which isn't a reasonable request. It isn't difficult to code it up yourself, but why do a bunch of work to emulate what the program will do for you? An easier route would be to use the parse tasks construct.

tell application "OmniFocus"
	tell default document
		set Val1 to "foo"
		set Date1 to "# 1d"
		set Date2 to "# 3d"
		set Date3 to "# 7d"
		set Task1 to parse tasks with transport text Val1 & Date1
		set Task2 to parse tasks with transport text Val1 & Date2
		set Task3 to parse tasks with transport text Val1 & Date3
	end tell
end tell

See the built-in help for "Processing mail messages into actions" for details on the format to use.