Site basic hours calculating wrong

I am trying to set up a time sheet to work out site basic hours, these are between 07.00 and 18.00, the problem I have is with the summing up, basically my problem is indicated below:-

If I work 06:00 to 17:00 this should be 10 hours site basic and 1 hour premium hours, but the below sum calculates the site basic hours to 11
Or if I work 09:00 to 20:00 this should be 9 hours site basic and 2 hour premium hours, but the below sum calculates the site basic hours to 11

=IF(AND(J36="",M36=""),"",(IF(AND(Table_Data!J1),"",IF(AND(J36<=7,M36>18),(18-7),M36-J36))))

Site basic hours 07.00-18.00 Monday to Saturday. Premium rates 18.00-07.00 Monday to Saturday

Hi Roy,

Please let us know that if the work is done during 07.00 and 18.00 how much should be the work hours
1. 10 site basic + 1 premium OR
2. 11 site basic ?
I believe that 07.00 to 18.00 is the standard work timing.

and what is the max site basic allowed work hours in a day?