Looks like the PHP support for date conversion works only with US style dates. That will affect all date input in Zenphoto.
If you use a US date does it show you the images older than the date? The date used is the 'mtime' value of the image which normally will be when it was uploaded.
EDIT: Having looked, The zenphoto dates are all using the ISO standard format which works just fine in PHP.
Also, a bug seems to have crept into the schedule content which will be corrected in tonight's build.