How to insert formatted date in VIM

Abstract: Remember to escape the percent sign

I find that I cannot simply prefix a r! before plain date command to insert a date in VIM today, When I tried to insert date with

:r !date +%u

I got an error, the reason lies in the percent sign, which stands for the current file in VIM ex-mode. For example, you can count the current file like this:

:! wc %

Thus to insert a formatted date, you have to run:

:r !date +\%u