This post will be updated, but for the mean time this should work:

find . -maxdepth 2 -type d -exec stat -c "%y %n" {} \; | sort | tail -n 20

Update 1:

This is much faster:

find . -maxdepth 2 -type d -printf "%TY-%Tm-%Td %TH:%TM:%TS %TZ %p\n" | sort | tail -n 20

🙂