How hard or how easy it is depends on a) how far back you want to do the historical data conversions, and b) how many countries/regions you want to do the conversions for.
Simplifying it one step and assuming that you want to convert UTC to US Eastern Time, the best way to do it would be to store the dates on which the daylight savings time changed into a table and use that table to decide how many hours to subtract to get the US Eastern time. The history of daylight savings time dates are available on the web - for example here.
Note a few things if your requirements are any more complicated than that:
a) Even in the USA, some states (Arizona) and some regions (parts of Indiana) do not observe daylight savings time.
b) The schedule of when the daylight savings time takes effect has changed historically. Currently it is the first Sunday of March (or something like that), but that was put in effect some ten years ago.
c) The dates on which daylight savings time takes effect changes from country to country, so if you had to make your queries work for multiple regions, you would need a table with the region info and the dates of daylight savings time.