Fix date on mac builds

date on mac takes a -r argument that expects seconds, date on linux
takes a -d argument that expects a date string.  Prefixing the date
string with @ makes date on linux treat the time as seconds, but is
incorrect for date on mac.

Test: m docs
Change-Id: Ic2a585eaac4c25b7b471caa581d7f25827c3a6b3