Not everyone agrees on an exact time, typically the viability of the fetus outside of the womb is the consideration.
This would mean a baby that would be just premature wouldn’t be aborted. As you move back the viability would end up varying for each pregnancy, which is why after a set point doctors are involved. They then make a medical judgement balancing the viability and safety to the carrier.
So there is no hard date. The insistence on getting one simplifies a complicated issue where nuance is important.
I’ve noticed that a lot of anti-abortion laws target doctors, specifically to make the fuzzy nature of the cuttoff difficult.
That requires more circuitry. Just have the computer check every 0.25s, and if you have depressed the button it is a 1, else a 0.