I found that subscribing to Wall Street Horizon (2.5usd) there are available upcoming earnings dates in TWS. They are also available for the API.
Though testing this shows it seems not really functional. The TWS data lacks dates for many stocks. The API only returns symbols, no detail of type of event or date, and it also returns maximum 50 symbols, where TWS shows 250 (I assume the first upcoming 250).
So not very functional, but may still may help some. Like Japanese stocks can be checked, and you may then enter manually in TWS to get accurate date. Haven't found Japanese earnings anywhere else on the web. If someone has, please share..
Have questioned API support about the problems with this feature, so will see if they have some clarifications.
