Hi @asp,
unfortunately the offset is restricted to expressions matching the regular expression [-+][0-9]+[mshdwMy] at the moment. Templating and variables in dashboards is not support either, which is a common request. There are several GitHub issues like #4489, that relate to that. Please feel free to open a feature request or add to the existing issues. In the meantime though, since the query is contained in the URL, it would be possible to create a small external website or tool that performs the date math an generate a url for a single timelion sheet from that.