How to convert dates into Julian dates?

Question

Devendra 2024-4-8，13:14

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https://ww2.mathworks.cn/matlabcentral/answers/2104336-how-to-convert-dates-into-julian-dates

评论： James Tursa 2024-4-8，18:07

I have read the dates in yyyymmdd format as follows;

dates = 20230409 20230414 20230504 20230514 20230519 20230608 20230613 20230723 20230817 20230827 20230901 20231006 20231011 20231026 20231115 20231125 20231205 20231210 20240129

I want to convert these dates into julian dates. Please suggest me how to do it?

I will appreciate your kind cooperation.

Deva

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Stephen23 2024-4-8，13:49

编辑：Stephen23 2024-4-8，14:26

Make sure that you import the data as DATETIME, then use:

https://www.mathworks.com/help/matlab/ref/datetime.juliandate.html

Do not import dates as scalar numerics whose digits just happen to look like those of some unrelated date units.

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Answer 1

the cyclist 2024-4-8，14:07

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在 MATLAB Online 中打开

Assuming your dates are currently stored as a numeric array, then

dates = [20230409    20230414    20230504    20230514    20230519    20230608    20230613    20230723    20230817      20230827    20230901    20231006    20231011    20231026    20231115    20231125    20231205    20231210     20240129];
gdt = datetime(dates,"ConvertFrom","yyyymmdd") % Gregorian
gdt = 1x19 datetime array
Columns 1 through 13

   09-Apr-2023   14-Apr-2023   04-May-2023   14-May-2023   19-May-2023   08-Jun-2023   13-Jun-2023   23-Jul-2023   17-Aug-2023   27-Aug-2023   01-Sep-2023   06-Oct-2023   11-Oct-2023

Columns 14 through 19

   26-Oct-2023   15-Nov-2023   25-Nov-2023   05-Dec-2023   10-Dec-2023   29-Jan-2024
jdt = juliandate(gdt) % Julian
jdt = 1x19
1.0e+06 *

    2.4600    2.4600    2.4601    2.4601    2.4601    2.4601    2.4601    2.4601    2.4602    2.4602    2.4602    2.4602    2.4602    2.4602    2.4603    2.4603    2.4603    2.4603    2.4603
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See the documentation for juliandate for details on the meaning of that output.

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Steven Lord 2024-4-8，14:48

在 MATLAB Online 中打开

all other 10 variables becomes zero.

Do those other elements become zero or are they simply displayed as zero because they're much, much smaller than the Julian dates?

J = juliandate(datetime('today'))
J = 2.4604e+06
x = 1e-4
x = 1.0000e-04
V = [J, x] % V(2) _displayed_ as 0
V = 1x2
1.0e+06 *

    2.4604    0.0000
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<mw-icon class=""></mw-icon>
y = V(2) % but it's not actually _stored_ as 0
y = 1.0000e-04

If so, changing the display format may be sufficient for your needs.

format shortg
V
V = 1x2
1.0e+00 *

   2.4604e+06       0.0001
<mw-icon class=""></mw-icon>
<mw-icon class=""></mw-icon>
format longg
V
V = 1x2
1.0e+00 *

                 2460408.5                    0.0001
<mw-icon class=""></mw-icon>
<mw-icon class=""></mw-icon>

Devendra 2024-4-8，15:07

编辑：Devendra 2024-4-8，15:26

Thanks for your kind help. Yes they are displayed as zero because they're much, much smaller than the Julian dates? Yes it displayed all the variables. I appreciate your kind support.

Deva

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Answer 2

James Tursa 2024-4-8，16:20

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https://ww2.mathworks.cn/matlabcentral/answers/2104336-how-to-convert-dates-into-julian-dates#answer_1438316

在 MATLAB Online 中打开

Based on your comment that you are expecting numbers in the range of 273,287,298, etc., if what you are after is actually "day of year" and not "Julian Date", then

dates = [20230409    20230414    20230504    20230514    20230519    20230608    20230613    20230723    20230817      20230827    20230901    20231006    20231011    20231026    20231115    20231125    20231205    20231210     20240129];
gdt = datetime(dates,"ConvertFrom","yyyymmdd") % Gregorian
gdt = 1x19 datetime array
Columns 1 through 13

   09-Apr-2023   14-Apr-2023   04-May-2023   14-May-2023   19-May-2023   08-Jun-2023   13-Jun-2023   23-Jul-2023   17-Aug-2023   27-Aug-2023   01-Sep-2023   06-Oct-2023   11-Oct-2023

Columns 14 through 19

   26-Oct-2023   15-Nov-2023   25-Nov-2023   05-Dec-2023   10-Dec-2023   29-Jan-2024
days(gdt - datetime(year(gdt),1,1)) + 1 % day of year, with Jan 1 being day number 1
ans = 1x19
    99   104   124   134   139   159   164   204   229   239   244   279   284   299   319   329   339   344    29
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I am aware that some fields of study refer to "day of year" as "Julian Date", but this is incorrect terminology.

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Devendra 2024-4-8，17:35

Thank you very much for your kind help.

Deva

James Tursa 2024-4-8，18:07

@Steven Lord I thought I remembered there was a function for this but couldn't find it in datetime methods. Didn't think to look in day( ) doc. Thanks for the update.

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