I have found that the following command is good for creating a new screen and running a shell command in it:
screen -dmS newscreen -- sh -c 'date; exec $SHELL'
However, what if I want to run the exact same shell command, but using an existing screen? I tried the following, but when I entered the screen I saw nothing printed:
screen -S existingscreen -X -- sh -c 'date; exec $SHELL'