The applescript support is good and works well.
In your code fragment you have not specified which task you are referring to.
An inbox task ? A project task ? If so which project ?
Whether inbox or project tasks, which task specifically ?
The following, for example, would at least conform to the structure of the dictionary:
Code:
tell application "OmniFocus"
tell default document
set var1 to start date of first inbox task
end tell
end tell