Time Difference Indicator based on Country

Has anyone ever built a custom function to calculate the time difference between here (Ireland) and the origin of a lead. For example a field on the record that populates upon creation to indicate how many hours of a time difference are between here and that leads country (e.g -6 hours).

I'd imagine the custom function would say 'if country= Spain, 'Time difference= +2'...

It would be really helpful for our sales reps to be able to sort their views based on this field. Just wondering if it's been done before and if anyone could provide a template?

All assistance appreciated.