TestKit: how to set the Clock?

Use case: I have am writing an implementation of IETF http message signatures. The spec has a lot of examples of signed HTTP messages signed with a validity period. To test the library I would like to set the Clock to a specific valid time and to invalid times, to check that the implementation is correct, i.e. that it fails a correct signature when the date is no longer valid.

I have spent at a few hours looking through the isses that led to the test runtime including the very nice documentation, looked at the source code, issues that led to it. But I could not find out how to do that... I may be missing something obvious...