Google Calendar should really have a widget that adds an automatically generated, but customisable, menstrual cycle day-tracker. It could have a 28-day overlay counter for those of us who take hormonal contraception or who always have dead-on clockwork 28-day cycles, and a one-click application for everyone else where we could select Day 1 and it would number the next 30 days, or however long your cycle was, so you knew roughly when/where you were. Then, of course, on your next Day 1, even if it falls on Day 17 rather than Day 29, you just click to re-designate it Day 1 and it numbers the following 30 days and so on. I would find that much easier to keep up with/be aware of than the current manual page-numbering system I have in my paper diary.
Why hasn't this happened yet? Or has it? Do you want to write the code for it, maybe not for Google Calendar but for some other equivalent program?
P.S. Here is a post I liked very much about how we should just get the fuck over ourselves and start talking openly about menstruation in society as a whole. I would not have necessarily made this a public post without having read that. But why filter this? If I'm not talking about the unpalatable wet and bloody details of menstruation, but rather the expediency of being able to keep track of it, then why be hushed about it? I have consequently decided it is Not Sensible to pretend that menstruation doesn't exist to the extent of not making public posts about it. Besides, I would quite like for someone to make code for a menstrual-cycle-tracking-widget happen!