To use the top chart  example: If it's 2:58PM in the
summer then look at the center column up to 2p, and then to the left
(Daylight Savings Time column), and change 1900 to 1958, or if needed change
the 7p to 7:58pm. The 1958 could be expressed as 1958 UTC (UTC has replaced
the term GMT, but GMT is still used), or as Zulu, Z, or Universal Time. If
seconds (59 for example) also need to be displayed then add colons, such as
19:58:59.
If there were no minutes then 2:00PM CST could be expressed as
Nineteen Hundred Hours.
2400 and 0000 represent exact midnight and are somewhat
interchangeable. If you are trying to express time up to midnight then 2359
or 2400 would be a good choice, to express a morning event then starting
with 0000, or 0001 would be a good choice.
Some amateur radio contests/events may effectively use both and
operate from 0000 to 2400. Stretching out UTC wouldn't be proper  but you
may find a bar open to 2600 hours.
Most of the time UTC is referenced and display in hundred hours.
The AM PM in the chart is listed just as an alternate to the same UTC time
(has no relation to the local time in London, which could be different).
To avoid using this chart then remember "CAST" (Central Add
Six for UTCTime), so add 6 to the current winter CST 24hour
time, if it's 5PM which is 17:00 local then add 6 which is 2300 UTC. During
Daylight Savings Time only add 5 which would be 2200 UTC.
To decipher UTC back to Central Standard Time then do the
reverse and subtract 6 (5 if DST) from 2300 which is 1700 which would 5PM.
One issue that occurs is as earth spins and it becomes the start of a new day at midnight (1201AM, 0001)
on Wednesday in London (Prime Meridian Line), it will still be Tuesday in Dallas for the next 5 or
6 hours till the earth's spin catches up.
So an example of this would
be if a meeting is scheduled in advance for Dallas at Tuesday at 8:00PM CST  then that
same meeting would also be at Wednesday 0200 UTC (2:00AM). This can be
visualized on the circle chart above. The CST has a six hour period
from 6PM to Midnight where the UTC date will be on the next day.
One
other side of the Prime Meridian Line the example would be backwards, a 2:00
AM local time Sunday meeting in Bangladesh (+6 hrs) would be on Saturday at
2000 UTC time (8PM UTC).
Download the rectangle chart in PDF form:
5 inch tall
6 inch
7 inch
8 inch
Download (save target as) the circle chart
HERE , resize if needed.
The ◄ Left & Right ► arrows
change the time zone.





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