Thanks for the try, but doing this I get the message:
#1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'Timestamp(6)' at line 2
I have also tried to use Timestamp(8) but get the same message (the current time in the VARCHAR is 8 characters long).
I think I will be forced to convert the VARCHAR into a TIME format in the PHP page and do the maths with the conversion. It will slow the site, but at least it will work ... sort of.