but (asdf:oos 'asdf:load-op :cl-postgres-glue) failed because there is no such package in postmodern
For one, it is called simple-date-postgres-glue, and it is inside of the simple-date.asd file, so you have to load the simple-date system before asdf even finds it.
Why loading simple-date after cl-postgres does not work I do not know. You can look at simple-date.asd, the code in there is simple and shouldn't be hard to debug.