Relative Selection on timestamps in Reports

Dear,

I would like to be able to make a report on opportunities which have been changed the last 7 days.

In order to do that i would need a relative selection on the field 'Changed On'.

Unfortunately, the data sources do not offer standard relative selections on this attribute (nor on created on, ...).

On other attributes such as 'Start Date' or 'End date' you have a lot of choices for relative selections.

There is a functionality to create your own relative selections, i have tried it, but it does not seem to offer what i need.

Does anyone know how i could solve this?

Thank you,

Regards