Are you talking about fertility tools like Flo, Kindara, or FF? Our bodies aren't clocks, even for people who have normal, predictable cycles.
If you know you consistently have 28 day cycles, and you're two days late, I would take a test!
As for me, my cycles are completely irregular and I have a hard time knowing from month to month what will happen. That's my story though, not yours. After about six months, the cycle tracking apps can give you an average cycle length and then you can relax a bit knowing what range to expect. I wouldn't be too worried at two days late though.
Def take a test! If it's bfn and you feel like you normally do before AF, wait a few more days and test again if AF hasn't shown up.