FYI. For some users after upgrading to 1809 Windows Security shows the incorrect definition update time in Security Centre. For me it shows a future time! The update time is shown correctly if you instead look under Windows Update History. This appears to be just a display bug and does not seem to impact functionality.
Its hard to find much information about this, but hunting around in Feedback Hub, I found acknowledgement of the problem by a MS Engineer saying it has been fixed in build 18267 and up (which I believe is Windows 10 19H1). I guess at some stage they may port this fix back to 1809.
It is very comforting having definition updates from the future as I guess infection should be almost impossible. 🙂