DISPLAY or something?

home/scripts/x-root-datetime.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+systemctl --user import-environment DISPLAY XAUTHORITY
 while true; do
 	sleep 1 && date +"<-- %A, %B %d -- %H:%M -->" | xargs -I% xsetroot -name %
 done

modules/dwm.nix

@@ -27,6 +27,7 @@ in
       script = builtins.readFile ../home/scripts/x-root-datetime.sh;
       wantedBy = [ "graphical-session.target" ];
       partOf = [ "graphical-session.target" ];
+      after = [ "graphical-session.target" ];
     };
 
     environment.systemPackages = with pkgs; [