How does menstruation announce itself? Is postponement possible?

Hello, I have to ask another question because it just won't leave me alone. Unfortunately, my girlfriend (30) is still delayed in getting her period. Her last cycles were as follows: 26 days, 29 days, 27 days, 31 days, 28 days, 29 days, 23 days. We are now on day 30 and still no sign…