digital milometer on astra td isnt working properly when the lights are on?

Question

air bag warning light stays on and screen showing date time temperature goes off aswell

Answer 1

Those digital displays are on a printed circuit board. If the error with the odometer and the other lights only occurs when the headlights are on, you probably have a short on the board.

That is probably a warranty item. Take it to the dealership and have them replace the display board at no charge to you.

Answer 2

for the mileometer try turning the dashboard lights intensity up this operates in a sliding switch right next to the headlamp switch

air bag or seat belt is in some way faulty perhaps just a sensor

and the date time and temperature could be caused by fitting an aftermarket stereo

hope this helps