[smooc]

Financial data deals with cutoff times and those happen in a timezone. Thus particularly in finance timezones are important. You are right though that internally everything should be UTC internally and only when required it gets transposed to the timezone. However many legacy financial systems assume a local time zone and not UTC and they do not expose that information in their data. Happy reconciliation :-).

[midasuni]

If the market opens at 9am New York time then that’s not the same as storing 1400 utc. An law could be passed to change the offset on a given day

You have to store time in the relevant geographic time zone. Something occurring at 1200UTC on dec 1st is not the same as something occurring at 1200Europe/London on dec 1st. The two times may coincide today, but if the U.K. government decides to implement daylight saving next week as a fuel rationing measure, then your 1200London event will have to be displayed at 1300UTC, but your 1200UTC event would be at 1200UTC.

[Kamq]

I think you mean 1300GMT, I don't think the UK owns enough of the world these days to futz with UTC.

[midasuni]

1200 London time, so 1100UTC with daylight saving yes.

Point remains, many events are linked to local civil time, which can change - and not just to daylight saving. The Line Islands in Kiribati shifted a whole day in 1994, same with Samoa in 2011. Have an event set for 0800 local on the first Monday of the year in Samoa which you stored as UTC, and your events would have broken. Unless you store the relevant data (it occurs on first monday according to the timezone that Samoa is in), then you will be wrong.

Now of course you can still get problems if a new timezone is created, as time is stored as Europe/London, if Scotland creates its own timezone in the future a meeting in Aberdeen would no longer be in Europe/London but in a new area Europe/Edinburgh, with different behaviour, but that’s far less common than DST changes or other date shifts.

All in all you should store the relevant datetime and zone, not just “normalise” it to UTC.

[Kamq]

Oh, shoot. I got the way GMT shifts wrong. That's embarrassing. Fair enough.

[steveBK123]

One edge case to this is I've worked on financial systems where we kept the Asian market systems in JST, while EU/MENA/Americas ran on UTC. As long as you aren't dealing with FX/Futures, markets are generally open no earlier than 7am and no later than 6pm. This worked nicely since it meant that the DATE in each regions system&local clocks agreed.