When does MtGox "volume" get reset? What time of day (in GMT) does the volume get reset to zero?

I'm trying to correlate this with the charts, however I'm unclear as to when this actually would occur
It is a moving sum, it sums up the volume for the last 24 hours of trade.
Bitcoin trading never closes, so the usual way of calculating volume as the sum of trades that happen between the open and close of the market doesn't apply.