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Converting days since 0-01-01 to Julian/Gregorian days - how can IDL recognise 0 AD? [message #89398]

Tue, 07 October 2014 06:21

Jasdeep Anand
Messages: 16
Registered: August 2011

Junior Member

I have data which has been given timestamps with the units: "days since 0-01-01 00:00", which I'm assuming means since January 1st, 0 AD. I'd like to convert this into a format I can actually use, preferably Julian days so IDL can recognise the date format when plotting.

From what I understand I can't use any of the default IDL programs for this task (JULDAY, GREG2JUL, etc) because of the fact that the year 0 AD doesn't exist (IDL's calendar instead goes straight from -1 to 1 to conserve leap years).

What should I do? I'm unsure of the timestamps used to calculate the data (all I have is the year and month) and I'll be comparing this with data from a separate dataset which does not have this problem. I don't think I can use -1 or 1 in this case as the year because the final Julian day will be wrong. Has anyone encountered this problem at all?

Thanks,

J. Anand

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Re: Converting days since 0-01-01 to Julian/Gregorian days - how can IDL recognise 0 AD? [message #89401 is a reply to message #89398]

Tue, 07 October 2014 10:29

Michael Galloy
Messages: 1114
Registered: April 2006

Senior Member

On 10/7/14, 7:21 AM, Jasdeep Anand wrote:
> I have data which has been given timestamps with the units: "days
> since 0-01-01 00:00", which I'm assuming means since January 1st, 0
> AD. I'd like to convert this into a format I can actually use,
> preferably Julian days so IDL can recognise the date format when
> plotting.
>
> From what I understand I can't use any of the default IDL programs
> for this task (JULDAY, GREG2JUL, etc) because of the fact that the
> year 0 AD doesn't exist (IDL's calendar instead goes straight from -1
> to 1 to conserve leap years).
>
> What should I do? I'm unsure of the timestamps used to calculate the
> data (all I have is the year and month) and I'll be comparing this
> with data from a separate dataset which does not have this problem. I
> don't think I can use -1 or 1 in this case as the year because the
> final Julian day will be wrong. Has anyone encountered this problem
> at all?
>
> Thanks,
>
> J. Anand
>

This is what CF conventions say about "year 0":

http://cfconventions.org/Data/cf-conventions/cf-conventions- 1.7/build/ch07s04.html

Mike
--
Michael Galloy
www.michaelgalloy.com
Modern IDL: A Guide to IDL Programming (http://modernidl.idldev.com)
Research Mathematician
Tech-X Corporation

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Re: Converting days since 0-01-01 to Julian/Gregorian days - how can IDL recognise 0 AD? [message #89445 is a reply to message #89398]

Fri, 17 October 2014 03:39

Valeri Golev
Messages: 1
Registered: October 2014

Junior Member

07 октомври 2014, вторник, 16:21:47 UTC+3, Jasdeep Anand написа:
> I have data which has been given timestamps with the units: "days since 0-01-01 00:00", which I'm assuming means since January 1st, 0 AD. I'd like to convert this into a format I can actually use, preferably Julian days so IDL can recognise the date format when plotting.
>
>
>
> From what I understand I can't use any of the default IDL programs for this task (JULDAY, GREG2JUL, etc) because of the fact that the year 0 AD doesn't exist (IDL's calendar instead goes straight from -1 to 1 to conserve leap years).
>
>
>
> What should I do? I'm unsure of the timestamps used to calculate the data (all I have is the year and month) and I'll be comparing this with data from a separate dataset which does not have this problem. I don't think I can use -1 or 1 in this case as the year because the final Julian day will be wrong. Has anyone encountered this problem at all?
>
>
>
> Thanks,
>
>
>
> J. Anand

Please, check

IDL> print, JULDAY(1, 1, 1, 0, 0, 0) ; Jan 01, 1 AD
1721423.5
IDL> print, JULDAY(12, 31, -1, 0, 0, 0); Dec 31, 1 BD
1721422.5

As you can see, the difference is exactly one day. It happens because there the year 0 AD in the calendar does not exists by definition!

Don't worry about .5 fraction, it comes from old astronomical tradition (Julian days start at noon).

Cheers,

Valeri

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