"Ahead of time" is an English idiom that is used when something is occurring before a specific moment or milestone in time. It can be used to indicate that something is happening before a scheduled event. In other instances, "ahead of time" can refer to some task being accomplished at a pace that's significantly faster than the time it was originally intended to take. The phrase is uniquely expressive because it implies that a person can somehow actually get ahead of the march of time, which is impossible.
Many phrases become popular in a culture even though their literal meaning may not match up with the intended meaning when spoken. Certain words, when spoken together, take on a meaning all their own through popular use. These words and phrases are known as idioms, and the English language has many idioms devoted to time and the relationship of human beings to it. One of the most popular of these time idioms is the phrase "ahead of time."
The most common use of this phrase refers to the practice of people doing something before the arrival of a certain scheduled time. As an example, consider the sentence, "The movie starts at 5:30, but we should really try to get there ahead of time so we can get good seats." In this sentence, the "time" in question is 5:30. By arriving "ahead" of that time, it means that the people will be at the movie theater prior to 5:30.
Another way in which this phrase is commonly used is in reference to something that is being done at a pace faster than was originally expected. When used this way, the idiom is very similar to the phrase "ahead of schedule." As an example, consider the sentence, "The factory workers were expected to take three days to complete this project, but they've been working so hard that they are well ahead of time." In this case, the "time" in question is the three days the project was supposed to take. By working so well, it's implied that the job will be done far before that three-day period has elapsed.
Of course, there is no way that anyone can actually get "ahead of time" in reality. The phrase uses this bit of metaphor as a way to say something in a familiar and colloquial way. It provides a short-hand, colorful method of indicating when something is happening before a predetermined time.