Time zones and convertor

Hi,
I have a simple spreadsheet with Cities, Times, Differences and Clock as headers.
I want to be able to calculate the city times across the world by using a "=NOW()" formula formatted to "HH:MM" as a BASE TIME, and adding / subtracting the time differences to give the local time.

BASE TIME 10:21
CITY ZONE TIME DIFFERENCE CLOCK
Makati City East PHT 8 10:21
Athens Europe EET 2 10:21
Genoa Europe CET 1 10:21
Ghent Europe CET 1 10:21
Rotterdam Europe CET 1 10:21
Stockholm Europe CET 1 10:21
London Europe GMT 0 10:21
Buenos Aires America ART -3 10:21
Houston America CDT -5 10:21
Concord America PDT -7 10:21

Any help would be appreciated.
many thanks

Jason