On 3 Oct 2018, at 14:26, Sven Van Caekenberghe <sven@stfx.eu> wrote:

Metacello new 
 repository: 'github://svenvc/ztimestamp';
 baseline: 'ZTimestamp';
 load.

This part is bad.
You need: 

Metacello new 
 repository: 'github://svenvc/ztimestamp/repository';
 baseline: 'ZTimestamp';
 load.

(You miss to point to the repository).
And btw, there is an error here: 

https://github.com/svenvc/ztimestamp/blob/master/repository/.properties

(File is malformed. It will not affect the loading, but you may want to fix it).

Esteban