The time limit isn't a 'bug', it's a 'bad feature'. And yeah, there are ways around it. Download a mod, or before starting a campaign, go into the 'historical battle' menu, but don't play a battle, then come out of the menu and make a new campagin. No time limit!

Also, I've never found taking a city within 20 minutes to be too difficult. Once it got a little close (when I assulted Rome) but maybe giving up was a bit premature? After all, if you fought there would be a chance of victory, but giving up assures there is none...