Melanie,

I need to keep the system running for at least 6 months +- a day or so.
The PLC is far from me (as in another country) so it is not easy or cheap to reset it or change the setting. My original idea was to have a DS1994 with it's alarm set at 6 months and another without an alarm. When the customer pays I ship a new DS1994 without alarm to them. As long as the clock ticks all is happy, if it gets removed for more than maybe an hour it will shut down too.
Maybe that is too complicated.....
Would two ibutton eeproms with different codes do the same thing if the PIC was the clock and I would have to ensure that that PIC clock works for years, it cannot fail after the customer has paid.

Regards

Chris