Any extensions that are made during the season are effective immediately. This means if you extend a player during the 11/12 season, the first year of the contract is 11/12.
Sorry this detail was missing from the rules.
Once the season is over there will be a lengthy extension period, from the end of the season until the end of the NHL entry draft. During this time you can extend players or trade player's negotiating rights. After this period there will be about a 2-3 week period to select your draft keepers, and following that we will have our supplemental draft.