I should have asked differently: Is offset to broker time determined at login only and then applied to timestamps returned by BrokerTime?
If there is an "offset", this depends on how the plugin writer wrote it.
As an example, I wrote the Ally Plugin. The time stamps were always in a known time zone - Eastern Time, with or without daylight savings. So I hard-coded it to correctly translate to UTC every time. This is a no-guesswork setup. I cannot comment on the MT4 setup.
