How to get seconds-since-epoch in shell script

[Mark-Jason Dominus]

I want to be able to get the the time in seconds-since-epoch
form, and I was surprised to learn that none of the `date'
commands we have near here had a seconds-since-epoch format,
since it would have been so easy to include.

Is there a way to get that time that is easier than
computing it from the second, minute, hour, julian-date, and
year?
--

 In some sense a stochastic process can do better; at least it has a chance.
Mark-Jason Dominus 	  			    mjd at central.cis.upenn.edu