For example I am doing an analysis on traffic across 20 countries. Currently our default timezone is set as EST. Is there a good and efficient way for me to do this analysis where 3 PM EST and 6 PM GMT will show up as 3 PM EST and 1 PM EST?
I am trying to do this in Big Query or another SQL based environment (so not the UI).
Do not forget that there are many countries with multiple time zones.
Not sure what data is available in your data set, but consider leveraging the time zone dimension if you have it. It's probably the only way to compute the time as per the described scenario.