Missing required field “updated”

Hi When testing my site I get the a.m. warning several times. When I looked at the list of fixes I saw that in an older version this warning was taken care of. But nevertheless I do receive it after the latest update. Can you please tell me how to fix this? What do I have to do and where do I find the solution? Somebody told me something about "time class" in function.php, but I cannot find it there. Many thanks in advance. MMchen