I don't think there is any correlation with level of education. I truly don't understand how people don't know this. There are a total of 12 months and the days per month absolutely never change outside of the occasional leap year in February.
What's wrong with counting on knuckles? You adjust your calendar with this contraption once per month.
> I truly don't understand how people don't know this.
And yet it's true. It would take me a lot of thinking to answer how many days are in an arbitrary month. And I'd probably only be 80% confident. I truly don't understand how I don't know this either. But I've never made an effort to learn it, nor has it been a problem.
Is it just once a month though? From my understanding the cal aligns day-of-week to day-of-month but there is no slider to adjust for 28 or 29 or 30 or 31 days.
Doing the knuckle dance every time you glance at it, to know whether to change the month already, seems annoying. (Yeah you'd memorize it for that month after a bit, but still you have to pay attention.)
What's wrong with counting on knuckles? You adjust your calendar with this contraption once per month.