In advance of current ideas, customs, or methods, as in His treatment of light showed this painter to be well ahead of his time, or Wearing trousers and smoking cigars marked Amy Lowell as a woman ahead of her time. This idiom uses time in the sense of "era" or "generation," a usage at least a thousand years old. The phrase is usually but not always used to express approval. [First half of 1900s]

Earlier, sooner, as in The meeting was scheduled for three o'clock, but most people arrived ahead of time. [Early 1900s]