To me, being of one's time implies a state of mind where one maintains some intellectual objectivity or fluidity about the relative importance of what one encounters in daily life. Being in one's time = incessant Twittering.
Evidence that you are in your time: having an abiding belief that your view of things is comprehensive, that your thinking is socially validated by the act of you having lived your thoughts, and that your thinking is or should be applicable to all. Evidence that you are of your time: having a belief that your view of things is subjective, that your view is contextually valid, and that how you think is not applicable to all.
Those who are in their time seem to get pleasure from reaction, identification, classification. Those who are of their time seem to get pleasure from reflection, connection, expansion.
If you are in this time, you pride yourself on knowing all the knowable things, and on placing new experiences into a category, based on your own previous personal experience. If you are of this time, you take pride in allowing for new things to integrate themselves into your understanding over time, through experience and experiment.
If you are in this time, the space you make in your head for new information is space that is partitioned from pre-existing rooms. If you are of this time, the new spaces you make tend to be additions -- built with no blueprint, and inconsistently sized.
And perhaps that is the distinction, in the end: is your mind is a solid house with uniformly-sized rooms, built from a plan, or is it an endless series of rooms to be discovered, explored, and furnished? Or perhaps the distinction is deeper still, more a matter of how fearful you are of unplanned additions, and how you respond to that fear.
To paraphrase Charles Sanders Pierce, do you allow your mind to adapt and let truth happen to an idea...or is living (and thinking) all about shooting it before it moves?