I want this job to execute at 9:55 tonight.

Normally, that would be simply:

at 2155
/bin/foo
^D

But, I want this one to execute in tmux, so I can keep an easier eye on it.

So, it's time to whip out the poor man's scheduler!

sleep 47400; /bin/foo

With #UNIX, there is always more than one way.