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Z Time Explained

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Z Time Explained

Surface weather map showing Z time

NOAA

Ever noticed several numbers followed by the letters "Z" or "UTC" listed at the top or bottom of weather maps, radar, and satellite images? Ever wondered what these mean?

This string of numbers and letters is a timestamp. It tells when the weather product was issued or when its forecast is valid. Instead of local AM and PM hours, a type of standardized time called Z time is used.

Z time vs. Military time

The difference between Z time and military time is so slight, it can often be misunderstood. Military time is based on a 24-hour clock which runs from midnight to midnight. Z, or GMT time, is also based on the 24-hour clock, however, its midnight is based on midnight local time at the 0° longitude prime meridian (Greenwich, England). In other words, while the time 0000 always corresponds to midnight local time no matter the global location, 00Z corresponds to midnight in Greenwich ONLY. (In the United States, 00Z can range from 2pm local time in Hawaii to 7 or 8pm along the East Coast.)

A Fool-Proof Way to Calculate Z Time 

Calculating Z time can be tricky. While it's easiest to use a table like this one provided by the NWS, using these few steps makes it just as easy to calculate by hand:

Converting Local Time to Z Time

  1. Convert the local time (12-hour) to military time (24-hour)
  2. Find your time zone "offset," or the number of hours your time zone is ahead of or behind local Greenwich Mean Time
     
    U.S. Time Zone Offsets
      Standard Time Daylight Saving Time
    Eastern -5 hrs -4 hrs
    Central -6 hrs -5 hrs
    Mountain -7 hrs -6 hrs
    Pacific -8 hrs -7 hrs
    Alaska -9 hrs  --
    Hawaii -10 hrs  --

  3. Add the time zone offset amount to the military time. This is the current Z time.
Converting Z Time to Local Time
  1. Subtract the time zone offset amount from the Z time. This is the current military time.
  2. Convert the military time (24-hour) to local time (12-hour).

Remember, in the 24-hour clock 23:59 is the final time before midnight, and 00:00 starts the first hour of a new day.

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