In the January 1973 issue of AA, Edward Lyndoe gave the time for the opening of the London Stock Exchange as May 18, 1801, 1:30 PM GMT, London. This is not for when it opened, but for the laying of the first stone of the building. He used it for mundane predictions. I have no experience with it in practice.
Lyndoe didn't give his actual sources, but summarized:
FWIWFinding one's way amid the data for this occasion (some tabulations being unbelievably inaccurate) tee emerges a chart which must surely be the ultima ratio for any critics of our subject. It so accurately represents the Exchange.