Currently, {{REVISIONTIMESTAMP}} returns the current timestamp (i.e., the same as {{CURRENTTIMESTAMP}}) while {{REVISIONTIMESTAMP: {{FULLPAGENAME}} }} returns the timestamp of the last edit of the page being edited.
See:
https://en.wikipedia.org/wiki/Wikipedia:Sandbox?oldid=595173679&action=edit&preview=yes
Version: unspecified
Severity: normal
URL: https://en.wikipedia.org/wiki/Special:ExpandTemplates?wpInput=*%7B%7BCURRENTTIMESTAMP%7D%7D%0A*%7B%7BREVISIONTIMESTAMP%7D%7D%0A*%7B%7BREVISIONTIMESTAMP%3A+%7B%7BFULLPAGENAME%7D%7D+%7D%7D%0A&wpContextTitle=Foo