timetext: Only allow positive times to protect against clock skew.
timetext: Only allow positive times to protect against clock skew.

If the server-included timestamp happened to be in the future of the
client's local time (due to clock skew), we'd end up with a strange
timestamp like "-1 year ago".

Fixes #7.

madbook/reddit-plugin-liveupdate,florenceyeun/reddit-plugin-liveupdate,madbook/reddit-plugin-liveupdate,sim642/reddit-plugin-liveupdate,florenceyeun/reddit-plugin-liveupdate,sim642/reddit-plugin-liveupdate,florenceyeun/reddit-plugin-liveupdate,sim642/reddit-plugin-liveupdate,madbook/reddit-plugin-liveupdate