Definition: the cumulative profit and loss from the close of the previous trading day to the present time
Today's P/L of a Position = (Today's Market Value - Yesterday's Market Value) + (Today's Selling Value - Today's Buying Value)
Today's Market Value is the real-time market value of the position; Yesterday's Market Value is the previous closing price for the regular trading hours multiplied by yesterday’s position size.
Today's P/L of an Account = Sum of Today's P/L of All Positions
Today's P/L is reset: at 12:00 a.m. local time
E.g. In the Australian market, Today's P/L is reset at 12:00 a.m. AET.