Paul, you are correct.
He started with the wrong formula from DateCalc.
He was using the one for Month and Year but wanted to include the day. That is also found in your work.
Here it is formatted the way he wants it:
Code:
{QUOTE
{SET Delay 1}
{SET od{DATE \@ d}}
{SET oy{DATE \@ yyyy}}
{SET mm{DATE \@ M}}
{SET ld{=(mm=2)*(od=28+((MOD(oy,4)=0)+(MOD(oy,400)=0)-(MOD(oy,100)=0)))}}
{SET yy{=oy+Delay}}
{SET dd{=ld*(28+(mm=2)*((MOD(yy,4)=0)+(MOD(yy,400)=0)-(MOD(yy,100)=0)))+od*(1-ld)}}
"{mm}-{dd}-{yy}" \@ "M/d/yyyy"}
Today, it returns 5/4/2021
This is formatted for US not European date format.
Here is a document containing the field.