Strange date fields in visualforce excel export

I want to render a visualforce page in Excel using contenttype = 'application/vnd.ms-excel#test.xls'.

This worked fine before except for some hidden coding in the top cells, but since a few months or so the system adds cells with week days, calendar months and year in the form of picklist to the end of the sheet. looks like unneeded html coding with a calendar picker is added. there is nothing in my page which calls these fields and they are not on the screen if I render the page regularly in html.

I saw in this board that other users have the same issues, but found no solution so far.