Computing elapsed time

[Greg Hill <ghill@synergymicro.com>]

The standard Emacs function current-time returns the time since 
00:00:00 1/1/1970 as a list of three 16-bit unsigned integers (msw 
lsw msec), where
	(msw * 2^16)   +   lsw
gives the number of seconds, and msec gives the additional number of 
milliseconds.

I need to compute the difference between two such timestamps as a 
single signed integer , accurate to the nearest second (ignoring the 
milliseconds is ok), and saturated to -134217728 to 134217727 seconds.

Does anyone have a function that does that?

Thanks.

--Greg