Measuring time in the future is horrendously inaccurate, as while you can take a pretty good guess at what it'll be (indeed the purpose of difficulty is to estimate and regulate this) between hashing power leaving and joining the network and the fact that the process is, well, random... it's just not a good way to measure time.

Measuring in the past is probably a lot more useful, because you have a globally agreed-upon timestamp. But what do you gain from it besides a little geeky fun and a lot of confusion?