There are some approaches you can take on passing the time.
1. Simply fast-forward or hand wave the time. Other posters covered this fairly well.
2. Each adventure also represents a level of time that is spent. This is my current approach to the campaign that allows the world around my players to progress along. For those of you who played in the RPGA such as Living Greyhawk will know these as Time Units. At the very beginning of each module, the players will know how much time will pass, which is usually one to two months of game time. This represents the time spent traveling to locations, gathering supplies, talking to other travelers, gathering information and relaxing. I also set aside time for those players who want to craft magic items. For example, if the module is one month, it actually is about two weeks of actually engaging the adventure and two weeks of doing [whatever] that the players want to do. This also helps you determine how other events are progressing forward. In my campaign, there is a civil war going on and it's been going on for about two years; however, it is starting to wrap up as one of the major Great Houses has been wiped out, but it progresses as time moves on.
3. Draw up your calendar and pin events on days ahead of time when they occur. Your PC's kick the bad guys out of town. The bad guys need help but it's going to take three weeks for that help to arrive. As you keep track of each day going by, you mark the date of when the reinforcements will arrive. If the PC's happen to be in town, then it's another showdown. If not, then set another date of when the PC's will get the word that the bad guys are back in town. This works well if you're meticulous in keeping track of a calendar, also the PC's could be in the middle of something else when this batch of trouble comes calling.
4. Go for the recurring villain scheme by skipping a module or two. In Module 1, the PC's drive out the bad guys from town. In Module 2, the PC's have to explore a ruin and kill a bunch of crazy cultists. In Module 3, the PC's are getting ready to embark on another adventure but are interrupted with the bad guys from Module 1 showing up. In Module 5, some of the crazy cultists from Module 2 come calling for revenge. The time that has pass will feel more real to the players because this is actual physical time that has passed when you reintroduce old villains from earlier modules. I did this as a way for PC's to think twice about unfinished business. The PC's were accused of being spies in a foreign country and were sentenced to be hanged by the baron who was out with his ranger, paladin, and other warriors. The PC's defended themselves by killing the ranger (who was the husband to the paladin), killing the warriors and chasing off the baron. They wounded the paladin, looted her, and tied her up. Well, one of the players was also a paladin and I ruled that he was behaving unpaladin-like. Later on, he got himself into some serious trouble with his order and was put on trial with the expected sentence of execution. Part of the trial was the other paladin testifying (the one they looted and let live) against the paladin.
Have fun!