[chongli]

Even questions like "What's the date of friday next week?" can't be quickly answered.

Fortunately we don't have a very pressing need to answer these sorts of questions; we have tools galore to handle them for us: smartphones, calendars, messaging apps, meetup etc. Topics like these often remind me of one of Einstein's more famous quotes:

[I do not] carry such information in my mind since it is readily available in books. ...The value of a college education is not the learning of many facts but the training of the mind to think.

We used to spend an enormous amount of time teaching our children mathematical tricks which have now all been made completely obsolete by the scientific calculator. Only the most rudimentary of arithmetical strategies have survived, if only for the fact that it's still a little inconvenient to reach into your pocket when scanning and counting objects with your eyes.

[rtpg]

I disagree, many times people are trying to coordinate and one person says the 8th thinking it's a friday, but it's actually thursday. So some people think they were talking about the 8th, others talk about it like it were the 9th, and there's confusion all about.

Casual calendar coordination is really easy to mess up in the current state.

[mburns]

That quote is about memorizing a relatively arbitrary number (5,280 feet to a mile) which is only relevant when you have systems that can't be easily reasoned about.

This is precisely why the metric system, or uniform length of months, are useful.