It was pointed out that the code I wrote doesn't always work in every locale depending on how your time is reported so I improved it. It'a still a command line cmd batch file:
for /f %%i in ('powershell "((get-date).Hour+18) %% 24"') do set startHour=%%i
for /f %%i in ('powershell "((get-date).Hour+12) %% 24"') do set endHour=%%i
reg add HKLM\SOFTWARE\Microsoft\WindowsUpdate\UX\Settings /v ActiveHoursStart /t REG_DWORD /d %startHour% /f
reg add HKLM\SOFTWARE\Microsoft\WindowsUpdate\UX\Settings /v ActiveHoursEnd /t REG_DWORD /d %endHour% /f
Again, save this code as a batch file and use Task Scheduler to run it every couple of hours to update your working hours.