org-display-custom-times

org-display-custom-times is a variable defined in `org.el'.
Its value is

nil

• Automatically becomes buffer-local when set.

Documentation:
Non-nil means overlay custom formats over all time stamps.
The formats are defined through the variable `org-time-stamp-custom-formats'.
To turn this on on a per-file basis, insert anywhere in the file:
#+STARTUP: customtime

You can customize this variable.