The Custom Variables appear at the bottom of the Variables page in the Store Editor. Custom Variables by default only include the Publish-Timestamp variable. However, you can also add custom variables that you can then call from any templates which you edit to include the custom variable.
Publish-timestamp: Displays the Unix time of the last publish. Store developers and designers may use this variable as a debugging tool to determine which pages have been published against the objinfo.xml file.
See Also