The PlainTasks plugin for Sublime Text is a terrific tool for working with plain text task management. It supports many aspects of the TaskPaper format and now adds relative due date parsing.
On any task, type d then ⇥ to insert an @due() tag with the cursor ready to enter a date. Type +3 and then ⇥ to set the due date to three days in the future.
The fun doesn't stop there. It supports lots of shortcuts like @due(+5) for 5 days in the future or @due(+3w) for 3 weeks in the future. You can also use shorthand for dates like @due(5) for the fifth of the current month (or the next month if the date has passed for this month).
There's also support for @started and @created tags with dates. What an impressive package for a text editor.