Does UTC observe daylight saving time?

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No, UTC itself never has DST. It is the constant frame of reference other time zones are expressed relative to.

From the Wikipedia UTC page:

UTC does not change with a change of seasons, but local time or civil time may change if a time zone jurisdiction observes daylight saving time or summer time. For example, UTC is 5 hours ahead of local time on the east coast of the United States during winter, but 4 hours ahead during summer.

In other words, when a time zone observes DST, its offset from UTC changes when there’s a DST transition, but that’s that time zone observing DST, not UTC.

Without knowing much about PHP time zone handling, it seems strange to me that you can specify “with DST” or “without DST” in a conversion – the time zones themselves specify when DST kicks in… it shouldn’t have to be something you specify yourself.

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