Mine changes if i don’t start within three days. After three days it will say 1 day late! I think because cycles can vary about a few days. If you’re TTC I would take a test! I use Flo and the Premom app
If the day comes that it predicted and you don't log that you started it then the app will change the day to the next day till you log it It won't assume you started and unless you have it in the settings that you are trying for a baby it won't say that you are late
I don’t rate Flo at all
I use pre mom app