I am on Yosemite, latest 5 image, lastest VM and using this code

ZnClient new url: 'https://github.com/kilon/ChronosManager/archive/master.zip'; setIfModifiedSince: (Date year: 2015 month: 12 day: 1); downloadTo: FileLocator imageDirectory.

it downloads the file but does not append the extension ".zip" as it should.

Unfortunately setIfModifiedSince: does not seem to be supported by Github :(