The Question is:
Does the LIB$DT_FORMAT logical
effect only DCL output?
iow, I'm wondering if any LIB$ (or SYS$)
date/time routines can also change
their output formats, per LIB$DT_FORMAT.
The Answer is :
Some do change formats, and some displays do not.
You might wish to read the OpenVMS RTL Library (LIB$) documentation
and particularly the time and timekeeping section of the OpenVMS
Programming Concepts manual, as you will find those to be excellent
resources for satisfying your curiousity.
Also see the OpenVMS documentation set the C Run-time Library
Utilities Reference Manual. (This manual and the C Run-Time
Library Reference Manual are now both considered part of the
OpenVMS documentation set.)