Using ISO date format in letter class

[Haines Brown]

I have a lualatex document that uses the letter class. I find that 
this class automatically inserts the state to the right following the 
senders' mame and addrress.

The date is displayed default with the format July 11, 2022.

I prefer the date be closer to the ISO standard. I want it to be like 
the output of

  \def\mydate{\leavevmode\hbox{\the\year.\twodigits\month.\twodigits\day}}
  \def\twodigits#1{\ifnum#1<10 0\fi\the#1}
 
  which is 2022.07.11

I tried placing \newcommand*\today{\number\year.\number\month.\number\day}
into the preamble, but lualatex complains that \today haa already 
been defined.

Haine Brown