Formatting Timestamp in Default Constraint
Can you format a TIMESTAMP WITH TIME ZONE column in a CREATE TABLE default constraint in Postgres v10.x?


No because that doesn't make any sense. Timestamps are stored as timestamps. It's a special type. You format things for display. Any point in time is stored the same way regardless. Formatting time should arguable not be done in the database at all.

Think of it like this, you may need Roman Numeral representation for some columns, but even if you do the database will stored integers in their binary representation.

Now if you want to change the clients default output formatting for timestamps, you can certainly do that.  From the docs


  The date/time style can be selected by the user using the SET datestyle command, the DateStyle parameter in the postgresql.conf configuration file, or the PGDATESTYLE environment variable on the server or client.