When does a period officially begin? And should you adjust timeframe each month?

When does Glow consider a period to have officially started? For example, I started spotting today, only when I wiped, and expect a full flow tomorrow. Do I say that my period starts tomorrow, and then report spotting for today? Or did it actually begin today? The other part of this question is whether you have to adjust the average length of your cycle in the settings each month? For example, my period is normally 23 days but depending on your answer above it may actually be either 24 or 25 for this month. Should I update this in the overall settings or keep it at 23 and maybe I just had an extra day this month? Any clarification would be much appreciated!