Forward-port no_gmt_unix_time fixes to 1.0.2 and beyond

As requested by Ben, I'm forward-porting the changes from no_gmt_unix_time (pull request #23) to 1.0.2 and master.  This branch merges cleanly to 1.0.2 and 1.0.2-aead, and its two commits cherry-pick cleanly to master.  They include another fix that was not in the original branch; I'm opening a second pull request for that one.