My period still hasn't turned up.

I haven't done another test , thought I'd wait until later in the week and test or go to the gp.

But in the meantime, if for some reason I have just skipped AF, how should I work out my cycle. e.g. what day do I count as Day 1?

I have never missed a period before, ever. And have been back to a regular cycle for the last 6 months so this is all new to me


Thanks!