[O] How to add "days since" in heading

[Ken Mankoff]

Hello,

I'd like to have something like "diary-anniversary" but report "days since" a 
date, rather than "years since". For example, I have

> ** Birthdays
> %%(diary-anniversary 12 11 2010) Their %d%s birthday

And in my Agenda I then see:

> events:     Their 21st birthday

I'd like to have something like this:

> ** Foo
> %%(diary-foo 2021-02-13) Bar [%d days]

And in my Agenda I then see:

> events:     Bar [4 days]


That is, I would like to track how long since an event (in this case, how long 
a paper has been in review) without using SCHEDULING which makes things appear 
overdue if scheduled in the past.

Thanks,

  -k.