I think at the time of the referendum a date was not yet decided. It wasn't until Article 50 was passed a date was announced. At the 2017 election, parties pledged to get the UK out of the EU. It still hasn't happened. When and if still depends on a number of factors. The biggest factor, at the moment, is the upcoming General Election. If Conservatives win a majority, the withdrawal agreement should be completed but the Brexit process still continues in regards to trade deals and a few other things. If Conservatives do not win a majority then Brexit could be delayed much longer or not even happen.