Luckily one of Microsoft's support pages had a somewhat related discussion from which I could pull a guess about the information I might need to use. I scanned as far as an example IF function using a value called DATEVALUE and proceeded to experiment.
As you might be able to guess this is calculating a date within another time zone. Given that the countries in my sheet play with Daylight Savings, I've considered playing with additional IFs to define TIMEVALUES from Daylight Savings start/end dates - but those dates change from year to year! More fun in store :D