in linux terminal, how do I show the folder’s last modification date, taking its content into consideration?

Question

Something like:

find /path/ -type f -exec stat \{} --printf="%y\n" \; | 
     sort -n -r | 
     head -n 1

Explanation:

the find command will print modification time for every file recursively ignoring directories (according to the comment by IQAndreas you can’t rely on the folders timestamps)
sort -n (numerically) -r (reverse)
head -n 1: get the first entry