Work around LESS "transparent" -> "NaNNaNNaN" bug.

Fixes #5.
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.
Show full datetime info in timestamp title.

This is a partial fix for #6.