1 second in 100 days, that is a crystal of 10 MHz within 1 Hz. Because 100 days is 8 640 000 sec. That is for a crystal already a chalenge, without an oven this is impossible and even then.
I wonder how the clock maker tuned his clock, because to make something like that you must have a reference, time or frequency. I know, I made a precise 60 Hz "artificial mains" for the mechanical, synchronous motor driven clock in my SB-630 station console. And then you realise how precise the running speed of a clock must be, to keep it on time during a longer period of time. In the USA the mains frequency is corrected over longer times, just as the 50Hz in Europe. Never the less the time of a mains driven clock can be about 1 minute late or too early. But I did not like that.
To have a good reference I built a GPS-controlled 10 MHz oscillator. With that I can see how precise the time base of my digital frequency counter is. Well, 1 Hz precision on 10 MHz, 10^-7, is too difficult for the built-in TCXO.