a51291f87d
Right now we write output of `date -d @{{ timestamp }} to
the expiry file, and then attempt to comapre with timestamp.
However, output of `date -d` is datetime and not timestamp,
so these 2 things can not be properly compared. So image cache
was valid forever.
Change-Id: I42f5b43f09d3c530813dd7fd334eafce7a5eaf39