I asked this question on TMF and the general answer was that companies only announce xdividend dates when they report, so these can potentially vary from year to year. So it may be best to check just after the preeceding reporting date.
Press on the Dividend Dates tab at the top. (Sorry, I am aware that this site doesn't work with some browsers and will try to rectify this in due course).
I try to maintain accuracy but there may be some errors or omissions. Also note that I normally only update the information once per week (typically a Monday evening) and dividends declared since the last update won't be included.