It's a bit larger than a 555, but it should keep you within a small handful of microseconds per day until the aging effects start to add up which make getting more than a millisecond a year dicey, even if the thing doesn't die on you: https://www.ipgp.fr/~crawford/2017_EuroOBS_workshop/Resource...
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There's got to be some rubidium frequency standard that's a drop-in replacement for a 555 timer ;)
A chip scale atomic clock (with a handy 1Hz output) can be yours for barely over five thousand euros and half that if you buy 250 of them. https://www.microchip.com/en-us/product/csac-sa65
It's a bit larger than a 555, but it should keep you within a small handful of microseconds per day until the aging effects start to add up which make getting more than a millisecond a year dicey, even if the thing doesn't die on you: https://www.ipgp.fr/~crawford/2017_EuroOBS_workshop/Resource...
GNSS (GPS) is the typical standard used.