I struggled with this too and somehow came up with a solution (using mysql and grafana 7.0.3).
This only works for Table Series though, in Time Series the Time value is rigorously ignored- I dont know whats going on there.
My DB field is of type datetime and I just save the mysql current_timestamp().
In Grafana I select the column and cast $__time(), then override unit as From now.
Here is an example using the query-builder, Ive also tested with raw sql which also works fine:
