The time Value of an Option is an anticipation of an increase in intrinsic Value. Time Value diminishes with the passage of time, as the closer the Option gets to the Exercise date, the less likely it is that the price of the Underlying asset will exceed or fall below the Strike price for a Call option and Put option respectively.

(See Chapter 24 Hybrid securities of the Vernimmen)

To know more about it, look at what we have already written on this subject