Service 21 lamp disco 3

[discovery3cairns]

Hi

I have a service 21 and a clock symbol flash every time I start my 2008 disco 3. Any ideas what this means? I have only had the car for a few month so I'm still learning all the ins and out of the car.

[sniegy]

Service is due in 21 days, this will then turn to a
-ve number & become days past the service date.

Cheers