is one of the most botched thing in IIC.

e.g. if you look for the address of a common eeprom, you can find it a 0xa0 (which, of course, shall not be shifted) and in other places, you can find it as 0x50 (which, of course, must be shifted).

Find a well known device from the same manufacturer and see if that particular manufacturer uses the shifted or unshifted annotation of the address. I never shift the address, if the chip I use uses unshifted, I just declare the shifted (e.g. if a serial EE is listed as 0x50, I define it as 0xa0). Another way to find out if shifted or unshifted is that, often in the timing diagrams you can see what the shifted address is.

Obviously, if you define the RTC you give a link to as having an address of 0xd0, the address should not be shifted.

Erik