Market mechanics
The idea
FX activity shifts as major financial centres open and close. More activity can improve available liquidity, but news can still widen spreads. Gold products can have daily maintenance breaks. Weekend and holiday schedules depend on the instrument and venue, so a session clock is a planning aid rather than proof that an order will execute.
A worked example
A London-New York overlap changes in UTC during the weeks when the US and UK change daylight saving time on different dates.
Try it yourself
Use the session clock with a winter and summer date and compare the UTC times.
A mistake to avoid
Assuming one fixed UTC opening time works all year.