If the “+30” is constant, just add a calculated field to the with the formula being the date field you use to filter (say it’s “Field_you_use_to_filter”), with 30 added to it.
Then in the dashboard, keep that newly added field (we’ll call it “Calculated_FieldAdded”) hidden.
Set 2 filters on the Grid to be:
- Field_you_use_to_filter >= TODAY
- Calculated_FieldAdded <= TODAY