Suppose it is noon where you are and you proceed west — and suppose you could travel instantly to wherever you wanted.
Fifteen degrees to the west the time is 11 a.m., 30 degrees to the west, 10 a.m., 45 degrees–9 a.m. and so on. Keeping this up, 180 degrees away one should reach midnight, and still further west, it is the previous day. This way, by the time we have covered 360 degrees and have come back to where we are, the time should be noon again — yesterday noon. What happened?
We got into trouble because longitude determines only the hour of the day — not the date, which is determined separately. To avoid the sort of problem encountered above, the international date line has been established — most of it following the 180th meridian — where by common agreement, whenever we cross it the date advances one day (going west) or goes back one day (going east).
That line passes the Bering Strait between Alaska and Siberia, which thus have different dates, but for most of its course it runs in mid-ocean and does not inconvenience any local time keeping.
Astronomers, astronauts and people dealing with satellite data may need a time schedule which is the same everywhere, not tied to a locality or time zone. The Greenwich mean time, the astronomical time at Greenwich (averaged over the year) is generally used here. It is sometimes called Universal Time (UT).