You generally shouldn't use the relative forms of the dates in your document. Because as you suggest they are always calculated relative to the current time, so probably won't do what you want. Instead you probably want to insert dates in there absolute form. For example
Of course dates like that are no fun to type, but TaskPaper makes this easier by:
- Edit > Insert Date
- Or better Command-Shift-P then type "id" to select the insert date command.
- Or better yet Tag > Due (Due and Start tags are special in that when you use a command to add that tag you'll automatically get prompted for a date.
Note that when you are interacting with the date popup panel you can use relative dates like (tomorrow, etc). But an absolute date is inserted into your document.
The place where relative dates are better used is in searches. You can save a search that says
@due <=[d] today + 24h and then easily find all items due today.
So gnerally use absolute dates in your document. Use relative dates in saved searches. Also make sure to check out this tutorial that covers many of these issues: