VBA help calculating UK and US date/times

I attached your file. I did not add any macros but just added several columns to the 'Information From SLA table' worksheet
Columns G-K
All I did is set the value of column B (update time) in G and changed the cell format to number and 5 decimals
the header explains it.
Now all the timestamps are long numbers and if you calculate based on these the time works (I think)
Total time spent on Incident IM1086019 is 10 hours 46 minutes and 23 seconds, but I don't think that's correct since you have to take into account the days, but maybe it good, that you'll have to check yourself.

This is just to show that if you use the timestamd (date and time) as serial number you don't have to worry about the date format.
Hope this gives you some idea.