How do you calculate how late you are?

[Dolwar]

period due day 29. now day 37. so am i 8 or 9 days late?

I'd say 8. Day 30 was 1 day late

That was my thought as well. Trying to resist testing before day 10.

If your period is 9 days late surely it would show up by now

True but I've heard that a negative on day 10 is a true negative