Some boards will prompt you to press the key to enter the BIOS as the only option when the CMOS battery is flat. Whether or not you set the clock, you still have to enter the bios to boot.
The battery is a standard CR2032, so it’s easy to replace, but it’s not something that most people experience, so it’s not common knowledge.
Personally I went about 6 months doing the same thing before I even bothered googling “how much does a CMOS battery cost” because it was an old pc anyway.