printf (is "+" in the argument is an add or concatenation)

Hi there, I'm still a newbie in using terminal. Hope somebody can help......I'm quite confused does "+" mean concatenate the date or add days into date?

printf '\nIn 2007, %s \n\n' "$(date -d" 2007-09-01 17:30:24.000000000 UTC +
$(( 36 /60 /60 /24 / 2 )) days" '+%d %B')"