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After several hours of tinkering with your times I am now wondering if I understood your request correctly lol: The attached workbook requires the open and close times in columns D and F to be given in local times, not your time. The rest of what I did just reconciles those local times with your computer time in A1:B1.
Toward this end there is a column C now which shows the time differences as compared to UCT. You should enter that same difference for your own location in C3. Note that C3 will always display "UCT" but you can see the number you entered in the Formula Bar. You can hide column C thereafter.
You can also hide columns E and G. They show the converted opening and closing times. Observe that these are date/time values because with time alone you won't be able to calculate beyond midnight.
I have added a check on the open time to the formula in column H to display "Close" when the date in column E is a Sunday. The result will be less than accurate but you will probably be able to expand that formula to add more precision.