Some stop at 45 and 90 so you have no idea how much injury time is left anyway
Scoreboards aren't allowed to put up how much injury time has passed. Too much pressure on the ref, or some such other FA bollocks.
Some scoreboards have a fancy dan clock on them so you can work out how long is left until you concede a last minute loser.
We have the Rolex right next to it though, no need for a clock!