User's Manual Part 2

B-3
UT1 (based on rotation of the Earth) and UTC (based on man-made
instruments) may differ, but never more than 0.9 second. By agreement,
when the difference begins to reach this point, a “leap-second” is
introduced in the UTC. This occurs, on average, every 12 to 18 months.
Universal Coordinated Time may be referred to colloquially (and
historically) as Greenwich Mean Time (GMT). This village lies on the
Greenwich meridian (0° longitude) in England, and for years was the
point of reference for all other time zones. Pacific Standard Time, for
example, is 8 hours behind UTC (UTC-8).
UTC and GMT may also be called Zulu time. This is a common military
term, and is typically shown in 24-hour format, such as 1800Z (6pm).
Finding the Universal Coordinated Time
There are a variety of ways to obtain the UTC. By computer, radio, and
even telephone.
United States Naval Observatory
• Internet
http://tycho.usno.navy.mil
United States Government
• Internet
http://www.time.gov
National Institute of Standards and Technology
• Internet
http://physics.nist.gov/time
• Telephone (delayed by approximately 30 ms because of land line)
[phone] in Ft. Collins, Colorado (not toll-free)
[phone] in Kauai, Hawaii (not toll-free)
• Radio
WWV - 2.5 MHz, 5 MHz, 10 MHz, 15 MHz, 20 MHz
WWVH - 2.5 MHz, 5 MHz, 10 MHz, 15 MHz
UTC and VitalSense
When collecting time-sensitive data, specifically across time zones, it is
not advisable to change the clock during data collection. Within the
VitalSense Monitor, the real time clock is to be set to UTC. This insures
that the data is universally time-stamped, and can be recognized and
analyzed by scientists throughout the world without regard to time-
conversion or gaps in the data caused by clock changes.