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.