http://finance.yahoo.com/q?s=BTCUSD=X
EDIT: It's GMT. The interactive chart shows 12:00 AM GMT as time of opening.
But this is actually aggregation not reality. If u look at BTC-E, BitStamp or LocalBitcoin, trading takes place round the clock. There is a big chance that non-bitcoiner financial people may follow Yahoo chart and that would result in wrong calculation. Opening & Closing does not apply to crypto trading !!!
If you're really concerned about it, contact Yahoo and see what they say about changing it. I think that more information is a good thing, not a bad thing.