Fiz, obrigado pela ajuda
@Echo Off
Set "locn=%UserProfile%\Desktop\Texts\Projects\Project WallTime\Day and Night"
Set "rstr=Reg Add "HKCU\Control Panel\Desktop" /V Wallpaper /D "
Set "str1=%locn%\Day.png"
Set "str2=%locn%\Night.png"
Set /A "now=10%TIME:~,2%" 2>Nul
If %now:~-2% Lss 17 (%rstr% "%str1%" /F >Nul
) Else %rstr% "%str2%" /F >Nul
RunDll32 User32.dll,UpdatePerUserSystemParameters >Nul