Bids and offers start 36 hours before the actual real-time consumption or trading period. Up to four hours (pre-dispatch) before the trading period starts a new forecast price is calculated to guide participants in the market. From four hours to the start of the trading period every half-hour a dispatch price is calculated and communicated. One hour before the start of the trading period bids and offers can no longer be revised (with some exceptions) and the new prices reflect Transpower's adjustments in load forecasts and system availability.