The real-time-clock IC manufacturers, of which TI is one, seem to use structured-time encoding (seconds/minutes/hours/days/months/years) rather than #-of-seconds-offset encoding. Why?
I wrote a blog entry about this: http://www.embeddedrelated.com/showarticle/90.php
We recently had to start using a real-time-clock at my company, and came to the conclusion that our processor's software would be much simpler and more performant if we could get an RTC chip that just reads its time in seconds since the Unix epoch (Jan 1 1970 12:00:00GMT), rather than one that gives us calendar time.
Why aren't there many ICs that allows readout of time in seconds since the epoch?