Suppose you want to schedule a contact with a ham in another part of the world, and you tell him to meet you on 14021 khz at 8 pm Thursday January 20th, 2011. Unless he is in the same time zone or in one close to yours, then he may have trouble figuring out what time you will be on the air calling him
Take a look at a chart of local times to get an idea of the problem he may face. If he is in Africa, then how does he figure out what you mean by 9 pm on Thursday night? The way to avoid this problem is use Coordinated Universal Time (abbreviated UTC). This is the time standard by which the world regulates clocks and time.
Time zones around the world can be expressed as positive or negative offsets from UTC as in this list; UTC replaced GMT as the basis for the main reference time scale or civil time in various regions on 1 January 1972.
I am in the Eastern Time zone in Canada, in winter, UTC = EST + 5 hours and in summer, UTC = DST + 4 hours. So for our contact, I would tell the other ham that I would meet him on 14021 khz at 0100 UTC Friday January 21st, 2011. We would not be confused about what time to meet and if conditions are right, then we would be able to complete the contact as scheduled.
Moral of the story… use UTC time for ham radio, not local time, it helps to avoid time confusion.
Ref: Current US Time