Our dog is diabetic. We were told to inject insulin twice a day in 12 hr intervals. We do 8am and 8pm. If we wished to have an evening out would 7 pm cause a problem?
If you inject your dog twice a day it's probably a slow-acting insulin. It shouldn't be a problem to give an injection earlier once in a while, but I wouldn't give it more than 1,5 hours earlier. Make sure to give the usual amount and continnue with your usual schedule the next morning.
Humans usually use fast-acting insulin right before eating. If you were told to feed your dog right after (or 30 minutes after) the injections, do so when injecting him/her earlier, too. Fast-acting insulin can lower the blood sugar to dangerous levels if you don't eat the required amount of carbs after injecting. So add a little treat to the early meal to make sure your dog eats at the right time relative to the injection.