The suggestion of a DS1231 is a good one, but......

I've had dreadful luck with Dallas Semiconductor parts.
They've gotten a reputation for "cool stuff that blows
up in production volumes". I might suggest the Maxim
MAX705 reset chip, since it's available, cheap and
as far as I know, reliable.

Not to bash DS excessively, but I've just not had really
great luck with their stuff. Multiple part types, over
a period of years, at several different companies.

Be forewarned....

Mark G. Forbes
forbesm@csos.orst.edu
R & D Engineer