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Blog Home » Tech Tip: Julian Days Format

Tech Tip: Julian Days Format

Tech Tip: Julian Days Format

Julian Days

Julian Days” is a numerical timestamp format, calculated by Sea-Bird Scientific CTDs as the number of the days since Jan 1 of that year (e.g., noon on Feb 3 = 34.5). While this format is useful, it is usually more convenient to display data in a visual format such as “2020-02-03 12:00:00″. Learn how to convert Julian Days to standard formats using tools like SBEDataProcessing, Excel, and Matlab.

Exporting “Time, Elapsed: Julian Days” from CTD data provides a count of days since Midnight on Jan 1 of that year, as referenced to the CTD’s timestamp. Some examples:

  • Midnight on January 1st: timeJ = 1​
  • Midnight on January 2nd: timeJ = 2​
  • Midnight on February 1st: timeJ = 32​
  • Noon on February 1st: timeJ = 32.5​​

To convert to a standard time format with a deployment starting in 2020​

  • Sea-Bird Data Processing: Use Time conversion in ASCII out​
  • In Excel: 1/1/2020+timeJ-1​
  • In matlab: datenum(2020, 1, 1)-1+timeJ​
  • In Python: d = datetime.date(dyear, 1, 1) + datetime.timedelta(yday – 1)​
  • In R: time1 <- ISOdatetime(year, 1, 1, 0, 0, 0, tz=’UTC’) + (jday – 1)*86400​

However, beware that the timeJ is referenced against the timezone of the instrument which can either be local or UTC​

# start_time = Feb 25 2019 12:00:01 [Instrument’s time stamp, first data scan]​

June 1, 2020

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