Broker Server Time, Daylight Saving, and the Hour Your EA Quietly Loses

Every EA with a time filter is reading a clock, and that clock is not London's or New York's. It is the broker's server clock, and for most brokers that clock jumps by an hour twice a year. If the description says "trades 08:00–16:00 server time", the question that matters is which real-world hours those are in winter, which in summer.

A broker picks a fixed offset from UTC and then decides what to do about daylight saving. The most common convention is UTC+2 in winter and UTC+3 in summer, shifted on the US dates, because that keeps the daily bar closing at 17:00 New York and gives five daily candles a week. Others shift on the European dates. Some keep one offset all year and never move.

A filter written in server hours is pinned to the server, not to a session. When the broker follows the US dates and Europe changes two or three weeks apart, London opens an hour earlier or later on the server clock for those weeks, then snaps back. When the broker follows neither, the shift lasts the whole summer.

Compare the clock in Market Watch to UTC on a date in winter and again on a date in summer. If the difference is the same on both, the broker never shifts. If it differs by one hour, find the two dates on which it changed; those tell you whether it follows the US or the European calendar. History gives the same answer: look at the open time of a daily candle from January and one from July. This takes ten minutes and there is no substitute for it.

Historical bars are stamped in the server time the broker used at the time, so a backtest on that history runs under that convention automatically. A set file tuned on one broker's data has its hours tuned to that broker's clock. Loaded on a broker with a different convention, the inputs look identical and every session window is one hour off.

Session breakouts, Asian-range strategies, news filters keyed to clock time and anything with daily-close logic are all sensitive to the hour. Trend systems that read an indicator on the close of an H4 or D1 bar care much less about the hour of day, but they are not immune, because the bar boundary itself moves with the server offset. A daily candle that closes at 23:00 instead of 00:00 contains a different hour, has a different open, high, low and close, and so every D1-based signal changes a little.

After each clock change, open the trade history and check one trade's open time against the session the description says it should be trading. If it matches, nothing needs doing. If it is off by an hour and the EA has a GMT-offset or DST input, that input is the only thing to touch: change it by one and check again. Do not retune the session hours or the stops to fix an hour offset; the next clock change will undo the retune and leave you with two problems instead of one.

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