Warning – this makes reference to women’s issues! Read at your own risk!
I have trouble tracking dates sometimes, especially my period, so now I let Gmail do it for me. I used filters and email.
On the day of my period I send my gmail an email… eg. Subject: “[datetrack] Aunt flo visited”
Then I set up a filter in Gmail to screen all emails marked with “[datetrack]” to be marked read and to skip my inbox. That way I don’t have any embarrassing reminders about my period sitting there in my inbox.
Then when I need to recall when my last period was, I can just search my email messages for “aunt flo”. This is particularly useful for me as I can do this from my mobile phone. Handy in the doctor’s office.