[whpalmer4]

You can specify dates by entering things like "+1w" for one week from now, "+3d" for 3 days from now, "Tuesday +3w" for 3 weeks from Tuesday, etc. Not quite what you are requesting, but perhaps sufficient? OmniFocus doesn't have a notion of a "fuzzy" date, so an action or a project either has an exact date/time, or none at all, for start/due/completed. The shorthand I mentioned above at least saves you from having to figure out the what the date of "three weeks from Tuesday" is.

See "setting start dates" in the Help command for more examples.