Re: Oh dear
Isn't a drugstore Timex or equivalent quartz watch as accurate as an expensive chronometer of the old days, or more accurate? One could check it daily against a trusted clock, so that it would have to keep time independently only for the length of the outage.
Also, the US Hydrographic Office (now the Navy Oceanographic Office) used to publish big books of tables for just this use. I don't know where you'd get them now, but I bet the Navy does.