Problem solved - but - I don't know why.
Solution:- I changed the Date/Time format from UK to US, opened Event Viewer and it was as it should be. Date & Time showing.
I then changed format back to UK, opened Event Viewer and , as above, the Time was back up.
If anybody knows what had happened I would be most interested.
My thanks to Bill............. from another site for helping me out with this one.