An information

[chants]

If you dont need the clock to tick after boot then battery removal is th quick solution.

I think to not have to manually do it in BIOS since you say every boot back to same time, that well BIOS modding is the best but an overkill method. Not only reversing the BIOS understanding the time module, finding some space for your patch and such but also flashing it when it's not properly packaged or signed or what other restrictions are there. Any mistake and the machine is bricked, a laboratory must reflash the ROM.

If you are loading Windows then make a Kernel driver and set it to boot priority to make it early in the boot process. You might have to turn off driver signing enforcement if you do not have an authenticode certificate to sign it. Probably the driver will be very short on code but I do not know what kernel calls or effectively BIOS interrupts are needed to do it. Obviously you won't use WinAPI here.